Authentication fully broken since 4.4.0

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Steps to reproduce

1. Update to 4.4.0
2. Try to use the app which fails
3. Log off
4. Try to log in again

Expected behaviour

Login is possible

Actual behaviour

After the update...

...no connection was possible:

I decided to log off. When trying to login, nothing works. Webauth fails completely, username and password ends at or after 2FA step with an empty authentication screen again. I managed to login once but then I was logged in regularly, the authentication of the app which is the target is not achieved. Nextcloud also complained about an unsupported browser. It's just a total mess.

Server configuration

Web server: Nginx

Database: Maria

PHP version: 8.3

Nextcloud version: Latest stable

If you have access to your command line run e.g.:
sudo -u www-data php occ app:list
from within your Nextcloud installation folder

If you have access to your command line run e.g.:
sudo -u www-data php occ config:list system
from within your Nextcloud installation folder

Browser

Browser name: App!

Browser version: /

Operating system: Linux

Insert your browser log here, this could for example include:
a) The javascript console log
b) The network log
c) ...

No source file, test, or entry point is identified. First reproduce the failure after updating to 4.4.0, including username/password, 2FA, and WebAuth flows, and inspect the authentication behavior and available logs. Done means users can log in and authenticate the target app successfully after logging out.