as my upgrade from Buster to Bullseye did not work I am about to set up the FBx anew. So after getting the image, and going through the initial setup process, I uploaded the system backups.
Next I installed Radicale and then uploaded the radicale backup from Buster.
Now the web interface login gives me an error 500. Maybe a glitch due to the uploaded old data?
Running FBx 21.8 on Bullseye now.