As I am using an IMAP-Backend for authentication I want to be able to use mail addresses as usernames.

Using the cli interface it's no problem to add usernames with an @ sign, but unfortunately it's not allowed on the web interface.

Please change

const USERNAME_PATTERN = '[0-9a-zA-Z_][0-9a-zA-Z_.]{1,38}|[0-9a-zA-Z]';

in

app/Controllers/userController.php

to something more useful like maybe

const USERNAME_PATTERN = '[0-9a-zA-Z_][0-9a-zA-Z_.@]{1,38}|[0-9a-zA-Z]';

(Added only an @ in pattern.)