@jens I tried logging in with your instance and got an error as well. I'm unable to debug it right now to see what's going on, but I *think* it might be related to your instance being on Mastodon 4.0, which doesn't support one of the auth methods added on 4.3.0...
The web version of Phanpy supports it, but I might have to do some extra work to support it on the native one. I think I only tested with instances on 4.3.0+ so I didn't run into that while building it