I don't know what caused the problem.
I'm getting an Oops error.
[2021-03-06 01:08:37] flarum.ERROR: InvalidArgumentException: Cannot load XML: Start tag expected, '<' not found
in /var/www/html/vendor/s9e/text-formatter/src/Renderer.php:46
Stack trace:
#0 /var/www/html/vendor/s9e/text-formatter/src/Renderers/PHP.php(303): s9e\TextFormatter\Renderer->loadXML()
#1 /var/www/html/vendor/s9e/text-formatter/src/Renderer.php(66): s9e\TextFormatter\Renderers\PHP->renderRichText()
#2 /var/www/html/vendor/flarum/core/src/Formatter/Formatter.php(111): s9e\TextFormatter\Renderer->render()
#3 /var/www/html/vendor/askvortsov/flarum-moderator-warnings/src/Api/Serializer/WarningSerializer.php(47): Flarum\Formatter\Formatter->render()
#4 /var/www/html/vendor/askvortsov/flarum-moderator-warnings/src/Api/Serializer/WarningSerializer.php(32): Askvortsov\FlarumWarnings\Api\Serializer\WarningSerializer->format()
#5 /var/www/html/vendor/flarum/core/src/Api/Serializer/AbstractSerializer.php(95): Askvortsov\FlarumWarnings\Api\Serializer\WarningSerializer->getDefaultAttributes()
#6 /var/www/html/vendor/tobscure/json-api/src/Resource.php(169): Flarum\Api\Serializer\AbstractSerializer->getAttributes()
#7 /var/www/html/vendor/tobscure/json-api/src/Resource.php(75): Tobscure\JsonApi\Resource->getAttributes()
#8 /var/www/html/vendor/tobscure/json-api/src/Document.php(187): Tobscure\JsonApi\Resource->toArray()
#9 [internal function]: Tobscure\JsonApi\Document->Tobscure\JsonApi\{closure}()
#10 /var/www/html/vendor/tobscure/json-api/src/Document.php(186): array_map()
#11 /var/www/html/vendor/tobscure/json-api/src/Document.php(224): Tobscure\JsonApi\Document->toArray()
#12 /var/www/html/vendor/flarum/core/src/Api/JsonApiResponse.php(27): Tobscure\JsonApi\Document->jsonSerialize()
#13 /var/www/html/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(139): Flarum\Api\JsonApiResponse->__construct()
#14 /var/www/html/vendor/flarum/core/src/Api/Client.php(72): Flarum\Api\Controller\AbstractSerializeController->handle()
#15 /var/www/html/vendor/flarum/core/src/Forum/Content/Discussion.php(108): Flarum\Api\Client->send()
#16 /var/www/html/vendor/flarum/core/src/Forum/Content/Discussion.php(64): Flarum\Forum\Content\Discussion->getApiDocument()
#17 /var/www/html/vendor/flarum/core/src/Frontend/Frontend.php(63): Flarum\Forum\Content\Discussion->__invoke()
#18 /var/www/html/vendor/flarum/core/src/Frontend/Frontend.php(55): Flarum\Frontend\Frontend->populate()
#19 /var/www/html/vendor/flarum/core/src/Frontend/Controller.php(32): Flarum\Frontend\Frontend->document()
#20 /var/www/html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\Frontend\Controller->handle()
#21 /var/www/html/vendor/flarum/core/src/Http/Middleware/ExecuteRoute.php(27): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}()
#22 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ExecuteRoute->process()
#23 /var/www/html/vendor/flarum/core/src/Http/Middleware/ShareErrorsFromSession.php(57): Laminas\Stratigility\Next->handle()
#24 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ShareErrorsFromSession->process()
#25 /var/www/html/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(40): Laminas\Stratigility\Next->handle()
#26 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CheckCsrfToken->process()
#27 /var/www/html/vendor/flarum/core/src/Http/Middleware/ResolveRoute.php(67): Laminas\Stratigility\Next->handle()
#28 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ResolveRoute->process()
#29 /var/www/html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Laminas\Stratigility\Next->handle()
#30 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process()
#31 /var/www/html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Laminas\Stratigility\Next->handle()
#32 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process()
#33 /var/www/html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Laminas\Stratigility\Next->handle()
#34 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process()
#35 /var/www/html/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle()
#36 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process()
#37 /var/www/html/vendor/flarum/core/src/Http/Middleware/CollectGarbage.php(46): Laminas\Stratigility\Next->handle()
#38 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CollectGarbage->process()
#39 /var/www/html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle()
#40 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process()
#41 /var/www/html/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle()
#42 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrors->process()
#43 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle()
#44 /var/www/html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process()
#45 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process()
#46 /var/www/html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle()
#47 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process()
#48 /var/www/html/vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(42): Laminas\Stratigility\Next->handle()
#49 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Laminas\Stratigility\Middleware\OriginalMessages->process()
#50 /var/www/html/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle()
#51 /var/www/html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process()
#52 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle()
#53 /var/www/html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process()
#54 /var/www/html/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle()
#55 /var/www/html/vendor/flarum/core/src/Http/Server.php(42): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#56 /var/www/html/public/index.php(26): Flarum\Http\Server->listen()
#57 {main}