Just looked at the earlier log, when no other 3rd party plugins were enabled Discussion thumbnail brought down the forum too, and the logs were as follows
[2021-02-20 20:44:40] flarum.ERROR: TypeError: Argument 3 passed to Flarum\Api\Event\Serializing::__construct() must be of the type array, null given, called in /var/www/vendor/flarum/core/src/Api/Serializer/AbstractSerializer.php on line 110 and defined in /var/www/vendor/flarum/core/src/Api/Event/Serializing.php:56
Stack trace:
#0 /var/www/vendor/flarum/core/src/Api/Serializer/AbstractSerializer.php(110): Flarum\Api\Event\Serializing->__construct(Object(Flarum\Api\Serializer\DiscussionSerializer), Object(Flarum\Discussion\Discussion), NULL)
#1 /var/www/vendor/tobscure/json-api/src/Resource.php(169): Flarum\Api\Serializer\AbstractSerializer->getAttributes(Object(Flarum\Discussion\Discussion), NULL)
#2 /var/www/vendor/tobscure/json-api/src/Resource.php(75): Tobscure\JsonApi\Resource->getAttributes()
#3 /var/www/vendor/tobscure/json-api/src/Document.php(187): Tobscure\JsonApi\Resource->toArray()
#4 [internal function]: Tobscure\JsonApi\Document->Tobscure\JsonApi\{closure}(Object(Tobscure\JsonApi\Resource))
#5 /var/www/vendor/tobscure/json-api/src/Document.php(188): array_map(Object(Closure), Array)
#6 /var/www/vendor/tobscure/json-api/src/Document.php(224): Tobscure\JsonApi\Document->toArray()
#7 /var/www/vendor/flarum/core/src/Api/JsonApiResponse.php(27): Tobscure\JsonApi\Document->jsonSerialize()
#8 /var/www/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(139): Flarum\Api\JsonApiResponse->__construct(Object(Tobscure\JsonApi\Document))
#9 /var/www/vendor/flarum/core/src/Api/Client.php(72): Flarum\Api\Controller\AbstractSerializeController->handle(Object(Laminas\Diactoros\ServerRequest))
#10 /var/www/vendor/flarum/core/src/Frontend/Frontend.php(72): Flarum\Api\Client->send(Object(Flarum\Api\Controller\ShowForumController), Object(Flarum\User\User))
#11 /var/www/vendor/flarum/core/src/Frontend/Frontend.php(51): Flarum\Frontend\Frontend->getForumDocument(Object(Laminas\Diactoros\ServerRequest))
#12 /var/www/vendor/flarum/core/src/Frontend/Controller.php(32): Flarum\Frontend\Frontend->document(Object(Laminas\Diactoros\ServerRequest))
#13 /var/www/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\Frontend\Controller->handle(Object(Laminas\Diactoros\ServerRequest))
#14 /var/www/vendor/flarum/core/src/Http/Middleware/ExecuteRoute.php(27): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}(Object(Laminas\Diactoros\ServerRequest), Array)
#15 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ExecuteRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Closure))
#16 /var/www/vendor/flarum/core/src/Admin/Middleware/RequireAdministrateAbility.php(23): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#17 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Admin\Middleware\RequireAdministrateAbility->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#18 /var/www/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(40): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#19 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CheckCsrfToken->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#20 /var/www/vendor/flarum/core/src/Http/Middleware/ResolveRoute.php(67): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#21 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ResolveRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#22 /var/www/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#23 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#24 /var/www/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#25 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#26 /var/www/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#27 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#28 /var/www/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#29 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#30 /var/www/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#31 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#32 /var/www/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#33 /var/www/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrors->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#34 /var/www/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#35 /var/www/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))