After updating from 4.12.3 every message has following words in the first line: Das Grantlee-Design „“ ist ungültig. I don't find the message text an adequate position for error messages.
please screenshot. Or try to change theme.
Found the reason for this. The theme is lost during the update. When setting the theme again the message is gone. As in other cases another work around may be to delete some the configuration files after the update.