WebDAV sync suddenly stopped working on iPad

I have a NAS with a WebDAV server for syncing. It worked fine on my iPhone, iPad and iMac. But suddenly it stopped working on my iPad. I have checked settings in the app, restarted my iPad and reset network settings on my iPad. I even re-installed the Omnifocus app. Nothing helped. I keep getting errors (“Cannot connect to server”) when syncing on my iPad, but on my iPhone with exact the same settings it works.

What strikes me is that after re-installing the app, I have to enter the URL of the server again, but the app does not ask for login credentials. I also tried resetting the database, but that gives the same results but now I get an error “unimplemented (501) PROPFIND”.

I can’t understand why sync stopped working on my iPad while on the iPhone it is still working perfectly. Am I overlooking something?

My iPad runs iPadOS 17.0.3 and my iPhone iOS 17.0.3. On both devices the latest version of OmniFocus is installed.