I thought the 2.1.1.23 was a minor update so there should not have been any breaking changes. That's my assumption. Naturally Jolla decides on update policies and should any breaking changes be reflected by updating also second version number. Hmm, I just checked my other Jolla and it is running Iijoki (2.1.0.11). I think the one I updated to Jämsänjoki was running the same. Maybe updating the third version number reflects e.g. kernel module load order change which might mean that 3rd party TOHs need an software update.

This was the first time I tried backup and restore, worked fine. There are some settings that were not recovered, like how often e-mail client should check for new posts, but otherwise seems to be ok. All the more important settings, accounts, calendar, contacts etc are fine.