Login name not aligned with nextcloud login name

[FelixOliverLange]

Steps to reproduce

1. create a user with an uppercase letter in the username
2. create an app token for that user
3. attempt to log in via a desktop or mobile app, writing the username in lowercase
4. the login will fail, while switching to uppercase will fix that

Expected behaviour

Nextcloud handles logins in its own apps and the web UI with ignored upper/lower case. Thus, a user will expect to be able to log in as "tony_stark" instead of "Tony_Stark". While it is debatable rather this is a good approach to usernames per se, it is the approach taken by Nextcloud. This only becomes obvious when observing the "user" field in the app token creation view.

Actual behaviour

Login is rejected due to username "missmatch", even though the same typing of that username is accepted in the web UI and clients. This is particularly an issue with app/desktop clients: Existing connections will no longer work, as the username used for login cannot be altered. So the user will have to remove the connection and re-add it with the "correct" spelling. Which may, depending on the user, not be obvious. Users thus may proceed to disable TFO again, assuming it is broken. If this occurs on all clients, the user is effectively locked out from his account, unless he uses a recover code generated earlier!

Server configuration

Standard Nextcloud 16.0.0 installation with PHP 7.3 in a webspace, using the Apache-PHP runtime.