Radicale Bullseye issues with backup from Buster

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.

So I set up the FBx with Bullseye again, including Radicale but did not restore the backup of Radicale. It is working fine.
So the issue we have is that when restoring a backup from a previous version to the new version, in this case from Buster to Bullseye. Some configurations may have changed which are handled usually in the Debian upgrade process. This means some sanity check has to be added to the “restore” function in order to avoid uploading outdated configuration files. Ideally the migration should also take place.
Currently I am in the situation that I have backups of my freedombox from Buster but cannot use them as I do not know where I might run into issues. Thus, I am feeding it with fresh data, like setting it up for the first time.
This is not how it should work.