wrong mail sent and confirmation mail empty

with Symfony Mailer installed, I get following errors:

Notice: Undefined property: Drupal\symfony_mailer\Email::$subjectReplace in Drupal\symfony_mailer\Email->render() (line 471 of modules/contrib/symfony_mailer/src/Email.php).

Drupal\symfony_mailer\Email->render() (Line: 207)
Drupal\symfony_mailer\Mailer->doSend(Object) (Line: 139)
Drupal\symfony_mailer\Mailer->Drupal\symfony_mailer\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
Drupal\symfony_mailer\Mailer->send(Object) (Line: 258)
Drupal\symfony_mailer\Email->send() (Line: 109)
Drupal\symfony_mailer_bc\MailManagerReplacement->mail('user_registrationpassword', 'register_confirmation_with_pass', '1test@test.de', 'de', Array, 'info@xxx.de') (Line: 605)
_user_registrationpassword_mail_notify('register_confirmation_with_pass', Object) (Line: 316)
user_registrationpassword_form_user_register_submit(Array, Object)
call_user_func_array('user_registrationpassword_form_user_register_submit', Array) (Line: 114)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 52)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 592)
Drupal\Core\Form\FormBuilder->processForm('user_register_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object) (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 42)
Drupal\webform_product\RedirectMiddleware->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

from log:
xxx/user/register
Referrer xxx/user/register
Message Error sending email: An email must have a "To", "Cc", or "Bcc" header.

the register_no_approval_required mail is sent but empty.
And the register_pending_approval mail is sent too, but that does not make sense.