Preconditions:

Created content type with ezuser. Two languages exists in the system.

Steps:

1. Create content item with content type from preconditions.
2. After creating the content item, go to translations tab and try to add new translation: click [+] button, select a language for your new translation and language to base on (if you won't select second language, validation error for ezuser would appear before publishing) and click "Create"
3. Translate translatable fields and click Publish

Actual result:

There is an error: Type error: Argument 1 passed to eZ\Publish\Core\Persistence\Cache\AbstractInMemoryHandler::escapeForCacheKey() must be of the type string, null given, called in /var/www/ezplatformee/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Persistence/Cache/UserHandler.php on line 122

Expected result:

Translation is added.