Fingerprint registriation fails - resetting all fingerprints in Android 9.0 / LineageOS 16.0

2. January 2020 - Reading time: 2 minutes

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

/data/system/users/0/fpdata/user.db

rebooted and the registration of new fingerprints worked fine again.

Some additional notes:

• The file /data/system/users/0/fpdata/user.db appeared on that phone again after successfully registering new fingerprints. Looks like a newly generated fingerprint database.
• This file does not exist on another XA2, which has functioning finger prints as well. Looks for me like (and I think I've read something about) different fingerprint hardware in both phones which uses a different storage folder.
• This folder on another LineageOS 16.0 device (Samsung Galaxy Tab S2) exists, but is totally empty. Looks like if the password is stored elsewhere on this device...
• So far everything works fine. I'll update this post if something strange starts to happen. And I'll keep the backup as well ;-)

Edit: After a system upgrade registering fingerprints was not possible because of "dirty" fingerprint reader. I just deleted

/data/system/users/0/settings_fingerprint.xml

rebooted and the registration of new fingerprints worked fine again. As always: Do backups before messing around in your data / system partition!