Login doesn't work if the entered server URL redirects to another location

It’s probably something very obvious that I’m doing wrong here, but I can’t quite find it.

So I set up my own server and and reach it fine through the webinterface, even from my android device.

However, when I try to use the app (version 2.2.0, installed from f-droid), I get the "There is no user with this username and password" error message. In the server logs, I can also see Error#01 for the access to `/current/user`.

Sorry if this is too obvious and could be found in the documentation. I didn’t find it.