Clearing cache didn't help.
flarum.log shows the same error:
[2020-07-10 11:52:05] production.ERROR: InvalidArgumentException: Cannot load XML: Extra content at the end of the document
in /home/my/domain/public_html/vendor/s9e/text-formatter/src/Renderer.php:46
Stack trace:
#0 /home/my/domain/public_html/vendor/s9e/text-formatter/src/Renderers/PHP.php(303): s9e\TextFormatter\Renderer->loadXML('<t><p><p>Vakar ...')
#1 /home/my/domain/public_html/vendor/s9e/text-formatter/src/Renderer.php(66): s9e\TextFormatter\Renderers\PHP->renderRichText('<t><p><p>Vakar ...')
#2 /home/my/domain/public_html/vendor/flarum/core/src/Formatter/Formatter.php(80): s9e\TextFormatter\Renderer->render('<t><p><p>Vakar ...')
#3 /home/my/domain/public_html/vendor/flarum/core/src/Post/CommentPost.php(172): Flarum\Formatter\Formatter->render('<t><p><p>Vakar ...', Object(Flarum\Post\CommentPost), Object(Laminas\Diactoros\ServerRequest))
#4 /home/my/domain/public_html/vendor/flarum/core/src/Api/Serializer/BasicPostSerializer.php(44): Flarum\Post\CommentPost->formatContent(Object(Laminas\Diactoros\ServerRequest))
#5 /home/my/domain/public_html/vendor/flarum/core/src/Api/Serializer/AbstractSerializer.php(84): Flarum\Api\Serializer\BasicPostSerializer->getDefaultAttributes(Object(Flarum\Post\CommentPost))
#6 /home/my/domain/public_html/vendor/tobscure/json-api/src/Resource.php(169): Flarum\Api\Serializer\AbstractSerializer->getAttributes(Object(Flarum\Post\CommentPost), NULL)
#7 /home/my/domain/public_html/vendor/tobscure/json-api/src/Resource.php(75): Tobscure\JsonApi\Resource->getAttributes()
#8 /home/my/domain/public_html/vendor/tobscure/json-api/src/Document.php(187): Tobscure\JsonApi\Resource->toArray()
#9 [internal function]: Tobscure\JsonApi\Document->Tobscure\JsonApi{closure}(Object(Tobscure\JsonApi\Resource))
#10 /home/my/domain/public_html/vendor/tobscure/json-api/src/Document.php(186): array_map(Object(Closure), Array)
#11 /home/my/domain/public_html/vendor/tobscure/json-api/src/Document.php(224): Tobscure\JsonApi\Document->toArray()
#12 /home/my/domain/public_html/vendor/flarum/core/src/Api/JsonApiResponse.php(27): Tobscure\JsonApi\Document->jsonSerialize()
#13 /home/my/domain/public_html/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(111): Flarum\Api\JsonApiResponse->__construct(Object(Tobscure\JsonApi\Document))
#14 /home/my/domain/public_html/vendor/flarum/core/src/Api/Client.php(72): Flarum\Api\Controller\AbstractSerializeController->handle(Object(Laminas\Diactoros\ServerRequest))
#15 /home/my/domain/public_html/vendor/flarum/core/src/Forum/Content/Index.php(108): Flarum\Api\Client->send(Object(Flarum\Api\Controller\ListDiscussionsController), Object(Flarum\User\User), Array)
#16 /home/my/domain/public_html/vendor/flarum/core/src/Forum/Content/Index.php(74): Flarum\Forum\Content\Index->getApiDocument(Object(Flarum\User\User), Array)
#17 /home/my/domain/public_html/vendor/flarum/core/src/Frontend/Frontend.php(63): Flarum\Forum\Content\Index->__invoke(Object(Flarum\Frontend\Document), Object(Laminas\Diactoros\ServerRequest))
#18 /home/my/domain/public_html/vendor/flarum/core/src/Frontend/Frontend.php(55): Flarum\Frontend\Frontend->populate(Object(Flarum\Frontend\Document), Object(Laminas\Diactoros\ServerRequest))
#19 /home/my/domain/public_html/vendor/flarum/core/src/Frontend/Controller.php(32): Flarum\Frontend\Frontend->document(Object(Laminas\Diactoros\ServerRequest))
#20 /home/my/domain/public_html/vendor/flarum/core/src/Http/RouteHandlerFactory.php(38): Flarum\Frontend\Controller->handle(Object(Laminas\Diactoros\ServerRequest))
#21 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(65): Flarum\Http\RouteHandlerFactory->Flarum\Http{closure}(Object(Laminas\Diactoros\ServerRequest), Array)
#22 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\DispatchRoute->process(Object(Laminas\Diactoros\ServerRequest), Object(Closure))
#23 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/ShareErrorsFromSession.php(57): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#24 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ShareErrorsFromSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#25 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/SetLocale.php(50): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#26 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\SetLocale->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#27 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(37): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#28 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CheckCsrfToken->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#29 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(32): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#30 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#31 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(51): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#32 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\RememberFromCookie->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#33 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/StartSession.php(61): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#34 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\StartSession->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#35 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/CollectGarbage.php(46): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#36 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\CollectGarbage->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#37 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(28): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#38 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\ParseJsonBody->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#39 /home/my/domain/public_html/vendor/flarum/core/src/Http/Middleware/HandleErrors.php(57): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#40 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Flarum\Http\Middleware\HandleErrors->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#41 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#42 /home/my/domain/public_html/vendor/middlewares/request-handler/src/RequestHandler.php(84): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#43 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\RequestHandler->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#44 /home/my/domain/public_html/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#45 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePathRouter->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#46 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Middleware/OriginalMessages.php(42): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#47 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Laminas\Stratigility\Middleware\OriginalMessages->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#48 /home/my/domain/public_html/vendor/middlewares/base-path/src/BasePath.php(53): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#49 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/Next.php(61): Middlewares\BasePath->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\Next))
#50 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(84): Laminas\Stratigility\Next->handle(Object(Laminas\Diactoros\ServerRequest))
#51 /home/my/domain/public_html/vendor/laminas/laminas-stratigility/src/MiddlewarePipe.php(73): Laminas\Stratigility\MiddlewarePipe->process(Object(Laminas\Diactoros\ServerRequest), Object(Laminas\Stratigility\EmptyPipelineHandler))
#52 /home/my/domain/public_html/vendor/laminas/laminas-httphandlerrunner/src/RequestHandlerRunner.php(96): Laminas\Stratigility\MiddlewarePipe->handle(Object(Laminas\Diactoros\ServerRequest))
#53 /home/my/domain/public_html/vendor/flarum/core/src/Http/Server.php(42): Laminas\HttpHandlerRunner\RequestHandlerRunner->run()
#54 /home/my/domain/public_html/public/index.php(26): Flarum\Http\Server->listen()
#55 {main}
