I got an answer from upstream. https://github.com/borgbackup/borg/issues/9010
Basically, migrations between beta versions are not supported.This is something that the user might want to know when upgrading. (It is quite possible that this was indeed notified to me by apt and I overlooked the message.)
Unless maintainers figure out a way to migrate repos between those specific versions, of course.
No big deal. I'll recreate my repo from scratch. -- Jérôme
