OK, that's definitely more a reminder for myself, but maybe some visitor has the same problem: After doing nothing recognizable on a Sony Xperia XA2 all previously registered fingerprints seemed to be deleted and the registration of new ones failed every time with an error message (something like "registration failed, please try again or use another finger"). After doing a Nandroid backup with TWRP, I wiped the data partition and found out, that the error was gone. Since I didn't want to do the whole setup and app installation process again, I tried to figure out which file(s) in the data partition cause this error and the first try was - lucky me - successful. I just deleted (be sure to have a backup!!!) the file
rebooted and the registration of new fingerprints worked fine again.
Some additional notes:
Edit: After a system upgrade registering fingerprints was not possible because of "dirty" fingerprint reader. I just deleted
rebooted and the registration of new fingerprints worked fine again. As always: Do backups before messing around in your data / system partition!