Error while user creation

Hey, i got a Problem I can not create new Users.

The error message is the following.

Exception #1243258395 in line 179 of /html/website-2021/Data/Temporary/Development/Cache/Code/Flow_Object_Classes/Neos_Neos_Controller_Module_Administration_UsersController.php: The message body must be of type string, "NULL" given.

53 Neos\Flow\Mvc\Controller\AbstractController_Original::addFlashMessage(NULL, NULL, "OK", array|0|, 1416225561)
52 Neos\Neos\Controller\Module\Administration\UsersController_Original::createAction("andreas", array|2|, Neos\Neos\Domain\Model\User, array|1|, "")

I found a solution:

Adding the following:

Neos:
  Flow:
    i18n:
      defaultLocale: de
      fallbackRule:
        order: ['en']