Finally found some time this bright Sunday afternoon to update my FP2 to MM.
Since I had deactivated multiple system apps including Google App I made a backup first using TWRP without installation like described by @retsifp (using the signed TWRP image provided by @z3ntu).
Then I reactivated the Google App (as mentioned in post #2) to avoid the bootloop problem and updated it to the current release.
This, however, did only help partially: the FP2 wasn’t caught in the bootloop but the update canceled with the same error message @fj40heinz mentioned in post #55. So I reactivated all deactivated system apps again and updated them to the current version, which didn’t help either.
The solution was to reinstall FP OS version 1.13.0 again (OTA file via Updater) like suggested by @ChuckMorris and @anon12454812.
After that the update to 17.04.8 went smoothly. Next step will be to make another TWRP to be on the safe side.
So thanks to all of you (and - not to forget - to @paulakreuzer) for providing your insights and solutions for the issues related to MM update here in the forum. You all really saved my day!
Special thanks go to the guys at Fairphone for providing the Android 6 update in the first place.