Precondition failed when trying to connect with android app

Hi Community, I have an issue and I can’t find the solution:

my setting:
Nextcloud is installed in Version: 22.2.3 on a Raspberry Pi3B with “Raspbian GNU/Linux 10 (buster)”

I am exposing Port 442 via a “myFritz” share (a service from the manufacturer of the Fritz!Box AVM) which looks like that: https://raspi.-some-alias-.myfritz.net/nextcloud/index.php

Now everything was perfectly fine until I moved to a new place and I bought a new Fritz!Box.
I assigned the PI the same static IP it had before, I re-shared the port 443 via myFritz-share, so the address changed https://raspi.-new-alias-.myfritz.net/nextcloud/index.php

Of course, I changed trusted domains in config.php, removed the old letsencrypt-SSL certificates, acquired a new one for the new alias and configured apache2 accordingly.

Now I face the following situation:
I can access my Nextcloud instance from any browser (external IP) using the alias: absolutely no issues.
However: If I try to connect via android App (I tried, the original NextCloud App, as well as oCloud and Cotton Cloud) I either get the message “server not found” / “could not find host”.

If I use cotton cloud I do get the message: “Secure connection established” and I am asked to supply user-name and password.
If I enter the wrong credentials I get the correct message “wrong username or password.”
If I enter the right credentials, I get the message “Precondition failed”.

In the eventlog of Nextcloud I do see::
Login failed: ‘Username’ (Remote IP: ‘8x.xxx.xxx.220’) in case of wrong credentials

In case of correct credentials I do not see anything in the log, but I can actually see the session/device under security:
Mozilla/5.0 (Android) cottonCloud-android/2.12

And now I am asking for help as I’d like to get Nextcloud working properly again.
Thanks a lot.