Reproduction of the bug is simple with safekeep from a system running Debian 11 trying to backup a system running Debian 12.
The backup can not even start, just get "Exception ''restrict_path'' raised of class '<class 'KeyError'>'" from rdiff-backup on Debian 12. Full back-trace from safekeep in the upstream bug report here: https://github.com/rdiff-backup/rdiff-backup/issues/872
