I have the same Problem on my Fairphone 4 with update iodeOS v6.14 to v7.6
I have used fixes/Updater-v6-to-v7.apk · master · ota / ota · GitLab
this updates the updater, so the v7.6 update can be run, but then i’m in in a boot loop with these errors:
Reason: init_user0_failed
ERROR: recovery: public: 179_2 failed to mount /dev/block/volmgr/public: 179_2: Permission denied
Thanks to the initial post here: Does FP have an issue with OS updates? - The Products - Fairphone Community Forum i also have boot the old system, just by click on “Try again” many times again and again… After some retries it just switch the Slot from B back to A here… The old installation looks fine…
There is really no way without backup/reinstall/restore ? Because seedvault can’t restore everything 
The whole backup topic on Android is just a pain 
see: Feature request: Backup solution