Can't assign roles

Applepiee

I'm not sure when this problem started, as it used to work fine, but when assigning roles, I get an error.
POST https://[redacted]/api/users/111870

{
  "errors": [
    {
      "status": "500",
      "code": "unknown",
      "detail": "PDOException: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`f_[redacted]_db`.`group_user`, CONSTRAINT `group_user_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users_bkp_sept_26_2019` (`id`) ON DELETE CASCADE) in /home.md10/home/[redacted]/public_html/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:117\nStack trace:\n#0 /home.md10/home/[redacted]/public_html/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php(117): PDOStatement->execute(NULL)\n#1 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(458): Doctrine\\DBAL\\Driver\\PDOStatement->execute()\n#2 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(657): Illuminate\\Database\\Connection->Illuminate\\Database\\{closure}('insert into `gr...', Array)\n#3 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(624): Illuminate\\Database\\Connection->runQueryCallback('insert into `gr...', Array, Object(Closure))\n#4 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(459): Illuminate\\Database\\Connection->run('insert into `gr...', Array, Object(Closure))\n#5 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(411): Illuminate\\Database\\Connection->statement('insert into `gr...', Array)\n#6 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Query/Builder.php(2611): Illuminate\\Database\\Connection->insert('insert into `gr...', Array)\n#7 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(216): Illuminate\\Database\\Query\\Builder->insert(Array)\n#8 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(160): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attach(6, Array, false)\n#9 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(112): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attachNew(Array, Array, false)\n#10 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(127): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->sync(Array)\n#11 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Database/AbstractModel.php(58): Flarum\\User\\Command\\EditUserHandler->Flarum\\User\\Command\\{closure}(Object(Flarum\\User\\User))\n#12 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(360): Flarum\\Database\\AbstractModel::Flarum\\Database\\{closure}(Object(Flarum\\User\\User))\n#13 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(209): Illuminate\\Events\\Dispatcher->Illuminate\\Events\\{closure}('eloquent.saved:...', Array)\n#14 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Concerns/HasEvents.php(162): Illuminate\\Events\\Dispatcher->dispatch('eloquent.saved:...', Array)\n#15 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(692): Illuminate\\Database\\Eloquent\\Model->fireModelEvent('saved', false)\n#16 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(663): Illuminate\\Database\\Eloquent\\Model->finishSave(Array)\n#17 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(138): Illuminate\\Database\\Eloquent\\Model->save()\n#18 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(90): Flarum\\User\\Command\\EditUserHandler->handle(Object(Flarum\\User\\Command\\EditUser))\n#19 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\\Bus\\Dispatcher->Illuminate\\Bus\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#20 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\\Pipeline\\Pipeline->Illuminate\\Pipeline\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#21 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\\Pipeline\\Pipeline->then(Object(Closure))\n#22 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(76): Illuminate\\Bus\\Dispatcher->dispatchNow(Object(Flarum\\User\\Command\\EditUser))\n#23 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/UpdateUserController.php(70): Illuminate\\Bus\\Dispatcher->dispatch(Object(Flarum\\User\\Command\\EditUser))\n#24 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(96): Flarum\\Api\\Controller\\UpdateUserController->data(Object(Zend\\Diactoros\\ServerRequest), Object(Tobscure\\JsonApi\\Document))\n#25 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\\Api\\Controller\\AbstractSerializeController->handle(Object(Zend\\Diactoros\\ServerRequest))\n#26 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(65): Flarum\\Http\\RouteHandlerFactory->Flarum\\Http\\{closure}(Object(Zend\\Diactoros\\ServerRequest), Array)\n#27 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\DispatchRoute->process(Object(Zend\\Diactoros\\ServerRequest), Object(Closure))\n#28 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#29 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\SetLocale->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#30 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(31): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#31 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\CheckCsrfToken->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#32 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(55): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#33 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithHeader->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#34 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#35 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#36 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#37 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\RememberFromCookie->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#38 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#39 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\StartSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#40 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#41 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Api\\Middleware\\FakeHttpMethods->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#42 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#43 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\ParseJsonBody->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#44 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#45 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\HandleErrors->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#46 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#47 /home.md10/home/[redacted]/public_html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#48 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\RequestHandler->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#49 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#50 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePathRouter->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#51 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Middleware/OriginalMessages.php(41): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#52 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Zend\\Stratigility\\Middleware\\OriginalMessages->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#53 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path/src/BasePath.php(53): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#54 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePath->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#55 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#56 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(72): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\EmptyPipelineHandler))\n#57 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-httphandlerrunner/src/RequestHandlerRunner.php(95): Zend\\Stratigility\\MiddlewarePipe->handle(Object(Zend\\Diactoros\\ServerRequest))\n#58 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Server.php(44): Zend\\HttpHandlerRunner\\RequestHandlerRunner->run()\n#59 /home.md10/home/[redacted]/public_html/index.php(22): Flarum\\Http\\Server->listen()\n#60 {main}\n\nNext Doctrine\\DBAL\\Driver\\PDOException: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`f_[redacted]_db`.`group_user`, CONSTRAINT `group_user_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users_bkp_sept_26_2019` (`id`) ON DELETE CASCADE) in /home.md10/home/[redacted]/public_html/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:119\nStack trace:\n#0 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(458): Doctrine\\DBAL\\Driver\\PDOStatement->execute()\n#1 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(657): Illuminate\\Database\\Connection->Illuminate\\Database\\{closure}('insert into `gr...', Array)\n#2 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(624): Illuminate\\Database\\Connection->runQueryCallback('insert into `gr...', Array, Object(Closure))\n#3 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(459): Illuminate\\Database\\Connection->run('insert into `gr...', Array, Object(Closure))\n#4 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(411): Illuminate\\Database\\Connection->statement('insert into `gr...', Array)\n#5 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Query/Builder.php(2611): Illuminate\\Database\\Connection->insert('insert into `gr...', Array)\n#6 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(216): Illuminate\\Database\\Query\\Builder->insert(Array)\n#7 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(160): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attach(6, Array, false)\n#8 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(112): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attachNew(Array, Array, false)\n#9 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(127): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->sync(Array)\n#10 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Database/AbstractModel.php(58): Flarum\\User\\Command\\EditUserHandler->Flarum\\User\\Command\\{closure}(Object(Flarum\\User\\User))\n#11 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(360): Flarum\\Database\\AbstractModel::Flarum\\Database\\{closure}(Object(Flarum\\User\\User))\n#12 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(209): Illuminate\\Events\\Dispatcher->Illuminate\\Events\\{closure}('eloquent.saved:...', Array)\n#13 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Concerns/HasEvents.php(162): Illuminate\\Events\\Dispatcher->dispatch('eloquent.saved:...', Array)\n#14 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(692): Illuminate\\Database\\Eloquent\\Model->fireModelEvent('saved', false)\n#15 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(663): Illuminate\\Database\\Eloquent\\Model->finishSave(Array)\n#16 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(138): Illuminate\\Database\\Eloquent\\Model->save()\n#17 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(90): Flarum\\User\\Command\\EditUserHandler->handle(Object(Flarum\\User\\Command\\EditUser))\n#18 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\\Bus\\Dispatcher->Illuminate\\Bus\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#19 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\\Pipeline\\Pipeline->Illuminate\\Pipeline\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#20 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\\Pipeline\\Pipeline->then(Object(Closure))\n#21 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(76): Illuminate\\Bus\\Dispatcher->dispatchNow(Object(Flarum\\User\\Command\\EditUser))\n#22 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/UpdateUserController.php(70): Illuminate\\Bus\\Dispatcher->dispatch(Object(Flarum\\User\\Command\\EditUser))\n#23 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(96): Flarum\\Api\\Controller\\UpdateUserController->data(Object(Zend\\Diactoros\\ServerRequest), Object(Tobscure\\JsonApi\\Document))\n#24 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\\Api\\Controller\\AbstractSerializeController->handle(Object(Zend\\Diactoros\\ServerRequest))\n#25 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(65): Flarum\\Http\\RouteHandlerFactory->Flarum\\Http\\{closure}(Object(Zend\\Diactoros\\ServerRequest), Array)\n#26 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\DispatchRoute->process(Object(Zend\\Diactoros\\ServerRequest), Object(Closure))\n#27 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#28 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\SetLocale->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#29 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(31): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#30 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\CheckCsrfToken->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#31 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(55): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#32 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithHeader->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#33 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#34 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#35 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#36 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\RememberFromCookie->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#37 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#38 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\StartSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#39 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#40 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Api\\Middleware\\FakeHttpMethods->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#41 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#42 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\ParseJsonBody->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#43 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#44 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\HandleErrors->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#45 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#46 /home.md10/home/[redacted]/public_html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#47 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\RequestHandler->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#48 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#49 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePathRouter->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#50 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Middleware/OriginalMessages.php(41): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#51 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Zend\\Stratigility\\Middleware\\OriginalMessages->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#52 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path/src/BasePath.php(53): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#53 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePath->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#54 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#55 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(72): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\EmptyPipelineHandler))\n#56 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-httphandlerrunner/src/RequestHandlerRunner.php(95): Zend\\Stratigility\\MiddlewarePipe->handle(Object(Zend\\Diactoros\\ServerRequest))\n#57 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Server.php(44): Zend\\HttpHandlerRunner\\RequestHandlerRunner->run()\n#58 /home.md10/home/[redacted]/public_html/index.php(22): Flarum\\Http\\Server->listen()\n#59 {main}\n\nNext Illuminate\\Database\\QueryException: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`f_[redacted]_db`.`group_user`, CONSTRAINT `group_user_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users_bkp_sept_26_2019` (`id`) ON DELETE CASCADE) (SQL: insert into `group_user` (`group_id`, `user_id`) values (6, 111870)) in /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php:664\nStack trace:\n#0 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(624): Illuminate\\Database\\Connection->runQueryCallback('insert into `gr...', Array, Object(Closure))\n#1 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(459): Illuminate\\Database\\Connection->run('insert into `gr...', Array, Object(Closure))\n#2 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Connection.php(411): Illuminate\\Database\\Connection->statement('insert into `gr...', Array)\n#3 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Query/Builder.php(2611): Illuminate\\Database\\Connection->insert('insert into `gr...', Array)\n#4 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(216): Illuminate\\Database\\Query\\Builder->insert(Array)\n#5 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(160): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attach(6, Array, false)\n#6 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php(112): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->attachNew(Array, Array, false)\n#7 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(127): Illuminate\\Database\\Eloquent\\Relations\\BelongsToMany->sync(Array)\n#8 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Database/AbstractModel.php(58): Flarum\\User\\Command\\EditUserHandler->Flarum\\User\\Command\\{closure}(Object(Flarum\\User\\User))\n#9 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(360): Flarum\\Database\\AbstractModel::Flarum\\Database\\{closure}(Object(Flarum\\User\\User))\n#10 /home.md10/home/[redacted]/public_html/vendor/illuminate/events/Dispatcher.php(209): Illuminate\\Events\\Dispatcher->Illuminate\\Events\\{closure}('eloquent.saved:...', Array)\n#11 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Concerns/HasEvents.php(162): Illuminate\\Events\\Dispatcher->dispatch('eloquent.saved:...', Array)\n#12 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(692): Illuminate\\Database\\Eloquent\\Model->fireModelEvent('saved', false)\n#13 /home.md10/home/[redacted]/public_html/vendor/illuminate/database/Eloquent/Model.php(663): Illuminate\\Database\\Eloquent\\Model->finishSave(Array)\n#14 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/User/Command/EditUserHandler.php(138): Illuminate\\Database\\Eloquent\\Model->save()\n#15 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(90): Flarum\\User\\Command\\EditUserHandler->handle(Object(Flarum\\User\\Command\\EditUser))\n#16 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\\Bus\\Dispatcher->Illuminate\\Bus\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#17 /home.md10/home/[redacted]/public_html/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\\Pipeline\\Pipeline->Illuminate\\Pipeline\\{closure}(Object(Flarum\\User\\Command\\EditUser))\n#18 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\\Pipeline\\Pipeline->then(Object(Closure))\n#19 /home.md10/home/[redacted]/public_html/vendor/illuminate/bus/Dispatcher.php(76): Illuminate\\Bus\\Dispatcher->dispatchNow(Object(Flarum\\User\\Command\\EditUser))\n#20 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/UpdateUserController.php(70): Illuminate\\Bus\\Dispatcher->dispatch(Object(Flarum\\User\\Command\\EditUser))\n#21 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(96): Flarum\\Api\\Controller\\UpdateUserController->data(Object(Zend\\Diactoros\\ServerRequest), Object(Tobscure\\JsonApi\\Document))\n#22 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\\Api\\Controller\\AbstractSerializeController->handle(Object(Zend\\Diactoros\\ServerRequest))\n#23 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(65): Flarum\\Http\\RouteHandlerFactory->Flarum\\Http\\{closure}(Object(Zend\\Diactoros\\ServerRequest), Array)\n#24 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\DispatchRoute->process(Object(Zend\\Diactoros\\ServerRequest), Object(Closure))\n#25 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#26 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\SetLocale->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#27 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(31): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#28 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\CheckCsrfToken->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#29 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithHeader.php(55): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#30 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithHeader->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#31 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#32 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\AuthenticateWithSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#33 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#34 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\RememberFromCookie->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#35 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#36 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\StartSession->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#37 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Api/Middleware/FakeHttpMethods.php(29): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#38 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Api\\Middleware\\FakeHttpMethods->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#39 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#40 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\ParseJsonBody->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#41 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#42 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\\Http\\Middleware\\HandleErrors->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#43 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#44 /home.md10/home/[redacted]/public_html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#45 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\RequestHandler->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#46 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#47 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePathRouter->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#48 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Middleware/OriginalMessages.php(41): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#49 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Zend\\Stratigility\\Middleware\\OriginalMessages->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#50 /home.md10/home/[redacted]/public_html/vendor/middlewares/base-path/src/BasePath.php(53): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#51 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\\BasePath->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\Next))\n#52 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\\Stratigility\\Next->handle(Object(Zend\\Diactoros\\ServerRequest))\n#53 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(72): Zend\\Stratigility\\MiddlewarePipe->process(Object(Zend\\Diactoros\\ServerRequest), Object(Zend\\Stratigility\\EmptyPipelineHandler))\n#54 /home.md10/home/[redacted]/public_html/vendor/zendframework/zend-httphandlerrunner/src/RequestHandlerRunner.php(95): Zend\\Stratigility\\MiddlewarePipe->handle(Object(Zend\\Diactoros\\ServerRequest))\n#55 /home.md10/home/[redacted]/public_html/vendor/flarum/core/src/Http/Server.php(44): Zend\\HttpHandlerRunner\\RequestHandlerRunner->run()\n#56 /home.md10/home/[redacted]/public_html/index.php(22): Flarum\\Http\\Server->listen()\n#57 {main}"
    }
  ]
}
clarkwinkelmann

Applepiee is users_bkp_sept_26_2019 a table created by an extension or something you did yourself manually ?
If that table is the result of a manual manipulation, it would appear that you broke the constraints during that manipulation, which instead to link between Flarum tables actively used, link to a backup table that's actually not used by Flarum. In this case it seems the group_user expects the user to exist in users_bkp_sept_26_2019, while the real constraint is supposed to check in users table.
If that's the only broken constraint, you could fix it manually (do a backup first!). However if multiple constraints are broken I would suggest you do a data backup (without structure), re-install Flarum, then re-import the data in the fixed database.