update: I dont know why but "register" has been auto fix with a magic. 😃
return code 500 in http://forumurl/register.
But user already save into database.
img:

log:
[2019-09-05 01:45:08] production.ERROR: Swift_TransportException: Expected response code 220 but got an empty response in /var/www/html/svdhcn/forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:445
Stack trace:
#0 /var/www/html/svdhcn/forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(358): Swift_Transport_AbstractSmtpTransport->assertResponseCode('', Array)
#1 /var/www/html/svdhcn/forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(147): Swift_Transport_AbstractSmtpTransport->readGreeting()
#2 /var/www/html/svdhcn/forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/SendmailTransport.php(50): Swift_Transport_AbstractSmtpTransport->start()
#3 /var/www/html/svdhcn/forum/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mailer.php(65): Swift_Transport_SendmailTransport->start()
#4 /var/www/html/svdhcn/forum/vendor/illuminate/mail/Mailer.php(483): Swift_Mailer->send(Object(Swift_Message), Array)
#5 /var/www/html/svdhcn/forum/vendor/illuminate/mail/Mailer.php(259): Illuminate\Mail\Mailer->sendSwiftMessage(Object(Swift_Message))
#6 /var/www/html/svdhcn/forum/vendor/illuminate/mail/Mailer.php(183): Illuminate\Mail\Mailer->send(NULL, Array, Object(Closure))
#7 /var/www/html/svdhcn/forum/vendor/flarum/core/src/User/AccountActivationMailer.php(72): Illuminate\Mail\Mailer->raw('Hey test!\n\nSome...', Object(Closure))
#8 [internal function]: Flarum\User\AccountActivationMailer->handle(Object(Flarum\User\Event\Registered))
#9 /var/www/html/svdhcn/forum/vendor/illuminate/events/Dispatcher.php(379): call_user_func_array(Array, Array)
#10 /var/www/html/svdhcn/forum/vendor/illuminate/events/Dispatcher.php(209): Illuminate\Events\Dispatcher->Illuminate\Events\{closure}('Flarum\\User\\Eve...', Array)
#11 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Foundation/DispatchEventsTrait.php(35): Illuminate\Events\Dispatcher->dispatch('Flarum\\User\\Eve...')
#12 /var/www/html/svdhcn/forum/vendor/flarum/core/src/User/Command/RegisterUserHandler.php(114): Flarum\User\Command\RegisterUserHandler->dispatchEventsFor(Object(Flarum\User\User), Object(Flarum\User\Guest))
#13 /var/www/html/svdhcn/forum/vendor/illuminate/bus/Dispatcher.php(90): Flarum\User\Command\RegisterUserHandler->handle(Object(Flarum\User\Command\RegisterUser))
#14 /var/www/html/svdhcn/forum/vendor/illuminate/pipeline/Pipeline.php(128): Illuminate\Bus\Dispatcher->Illuminate\Bus\{closure}(Object(Flarum\User\Command\RegisterUser))
#15 /var/www/html/svdhcn/forum/vendor/illuminate/pipeline/Pipeline.php(104): Illuminate\Pipeline\Pipeline->Illuminate\Pipeline\{closure}(Object(Flarum\User\Command\RegisterUser))
#16 /var/www/html/svdhcn/forum/vendor/illuminate/bus/Dispatcher.php(98): Illuminate\Pipeline\Pipeline->then(Object(Closure))
#17 /var/www/html/svdhcn/forum/vendor/illuminate/bus/Dispatcher.php(76): Illuminate\Bus\Dispatcher->dispatchNow(Object(Flarum\User\Command\RegisterUser))
#18 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Api/Controller/CreateUserController.php(46): Illuminate\Bus\Dispatcher->dispatch(Object(Flarum\User\Command\RegisterUser))
#19 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Api/Controller/AbstractSerializeController.php(98): Flarum\Api\Controller\CreateUserController->data(Object(Zend\Diactoros\ServerRequest), Object(Tobscure\JsonApi\Document))
#20 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Api/Controller/AbstractCreateController.php(24): Flarum\Api\Controller\AbstractSerializeController->handle(Object(Zend\Diactoros\ServerRequest))
#21 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Api/Client.php(71): Flarum\Api\Controller\AbstractCreateController->handle(Object(Zend\Diactoros\ServerRequest))
#22 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Forum/Controller/RegisterController.php(60): Flarum\Api\Client->send(Object(Flarum\Api\Controller\CreateUserController), Object(Flarum\User\Guest), Array, Array)
#23 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/RouteHandlerFactory.php(40): Flarum\Forum\Controller\RegisterController->handle(Object(Zend\Diactoros\ServerRequest))
#24 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/DispatchRoute.php(67): Flarum\Http\RouteHandlerFactory->Flarum\Http\{closure}(Object(Zend\Diactoros\ServerRequest), Array)
#25 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\DispatchRoute->process(Object(Zend\Diactoros\ServerRequest), Object(Closure))
#26 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/ShareErrorsFromSession.php(58): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#27 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\ShareErrorsFromSession->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#28 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/SetLocale.php(51): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#29 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\SetLocale->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#30 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/CheckCsrfToken.php(33): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#31 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\CheckCsrfToken->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#32 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/AuthenticateWithSession.php(34): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#33 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\AuthenticateWithSession->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#34 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/RememberFromCookie.php(52): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#35 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\RememberFromCookie->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#36 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/StartSession.php(62): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#37 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\StartSession->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#38 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/CollectGarbage.php(48): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#39 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\CollectGarbage->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#40 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/ParseJsonBody.php(29): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#41 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\ParseJsonBody->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#42 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithWhoops.php(43): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#43 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Flarum\Http\Middleware\HandleErrorsWithWhoops->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#44 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#45 /var/www/html/svdhcn/forum/vendor/middlewares/request-handler/src/RequestHandler.php(84): Zend\Stratigility\MiddlewarePipe->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#46 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\RequestHandler->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#47 /var/www/html/svdhcn/forum/vendor/middlewares/base-path-router/src/BasePathRouter.php(97): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#48 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\BasePathRouter->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#49 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Middleware/OriginalMessages.php(41): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#50 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Zend\Stratigility\Middleware\OriginalMessages->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#51 /var/www/html/svdhcn/forum/vendor/middlewares/base-path/src/BasePath.php(53): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#52 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/Next.php(60): Middlewares\BasePath->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\Next))
#53 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(83): Zend\Stratigility\Next->handle(Object(Zend\Diactoros\ServerRequest))
#54 /var/www/html/svdhcn/forum/vendor/zendframework/zend-stratigility/src/MiddlewarePipe.php(72): Zend\Stratigility\MiddlewarePipe->process(Object(Zend\Diactoros\ServerRequest), Object(Zend\Stratigility\EmptyPipelineHandler))
#55 /var/www/html/svdhcn/forum/vendor/zendframework/zend-httphandlerrunner/src/RequestHandlerRunner.php(95): Zend\Stratigility\MiddlewarePipe->handle(Object(Zend\Diactoros\ServerRequest))
#56 /var/www/html/svdhcn/forum/vendor/flarum/core/src/Http/Server.php(46): Zend\HttpHandlerRunner\RequestHandlerRunner->run()
#57 /var/www/html/svdhcn/forum/index.php(22): Flarum\Http\Server->listen()
#58 {main}
Thanks.