OCIS: RADICS3 CS3 Auth fails

I have a problem with the python auth code for radicale (Radicale v3 Documentation) at GitHub - dragotin/radics3: A CS3 Auth Plugin for the Radicale CardDAV/CalDAV Server which acts as a helper script to get credentials from OCIS/CS3 API into the radicale server. The setup worked for months but since a few weeks I get

Feb 03 20:08:04 radicale radicale[44999]: [44999/Thread-6 (process_request_thread)] [ERROR] An exception occurred during REPORT request on '/thommie/1d047ae2-0dac-3f42-470d-e7fb1a22a31c/': Failed to authenticate as user thommie: error authenticating user

The only workaround is a fallback with a local htpasswd file, as radicale does not support OAuth by itself. Can anybode support the debugging? OCIS version is 5.0.9