Notability WebDAV connection failed

I tried to connect notability, a note taking app for iPad, via WebDAV with my Nextcloud.
After trying to connect the error show in the app is:
“unable to parse xml Optional(”“)”.

My cloud runs on an Raspberry 5 with 8GB of ram.
The Version is:
I used the AIO-Docker container and a separate mounting point on an ssd that is connected via pcie.

The dav url I used (following the documentation from Nextcloud):

(where I replaced “” with my cloud URL and USERNAME with the username as whom I want to log into the Nextcloud.

Further the app asked me for the credentials and after trying some false passwords on purpose I can assume that I typed those correctly.
Further I tried to use an application specific password, this didn’t help eighter.

After reading this:

I assume I have to modify the apache somehow.
But I don’t know how to do that.

Nextcloud version :
Operating system and version : Debian GNU/Linux 12 - Raspberry Pi OS
Apache or nginx version : Apache/2.4.59
PHP version : 8.3.6