This is not restrained to non-X backups, unfortunately. I’ve been a Sailfish X user since June 2019, and after several delays, I gave in and decided to update from 22.214.171.124 to 126.96.36.199. I have more severe problems with this update than simply the backup, but from my experience, I completed a full re-flash of my phone with 188.8.131.52, then during the setup of the device, I skipped adding a Jolla account, and proceeded to restore my backup, which claimed to complete just fine.
Upon doing so, the Store app immediately complained about my account needing updating. I attempted to input my credentials, and it gives the error of “Unable to store app keys.” I removed my account completely, tried again, and then received a new error “Your username and password cannot be used on this device.” This was unfathomable, and while I was afraid it was a mistake in input, I proved that wrong by signing into account.jolla.com from within the Sailfish web browser.
The only fix was to re-flash again, and sign into my Jolla account during setup. This worked fine, but the backup had other problems when I did so. The Notes restore fails due to an ERROR 3 in /usr/bin/jolla-notes/notes-vault where it’s non-existent or non-executable. I removed my Notes app but the error persisted, thus halting the rest of the restore. I had to take the tar off my memory card, open it in 7zip, remove the Notes backup manually, put the tar back onto my phone, and THEN it would finally complete the restore correctly.
I have more serious complaints I’ll be making another post on, but this is a serious problem, either account keys don’t work, or parts of the backup fail, and this needs to be remedied.