Thanks bigslick202, this clue worked for me, on a Galaxy S6 where I still had access to the phone using a fingerprint, but had forgotten the backup password.
Short answer: Backup password is stored in Fingerprint app data cache. So just clear the Fingerprint app's data cache, and you'll be prompted for a new backup password.
Long answer, what I did was
- Go to the Application Manager\Fingerprint app
- 'Force Stop' and 'Clear Data'
- Reboot phone
- Unlock w/ fingerprint
- Go to 'Lock screen and security\Screen lock type' (which should be set to 'Fingerprints')
- Use fingerprint to get past the next screen, and
- Viola, the next sceeen prompts for a NEW backup password
- Now log that password down somewhere in your cloud storage!