Hello, I've investigated further (I've only now got the idea to try importing keyring in Python) and have discovered that what I have is bug #820905 in python-keyring. I don't remember creating the configuration file (maybe it was created automatically) mentioned there, but it did exist on my machine, and when I removed it, it started working again.
You can close this as a duplicate. Sorry for wasting your time. Thanks for uploading a new version of the extension though. Best wishes, Fedor Bug #820905: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=820905
