This week I had the same issue....but was never able to get back into my phone and had to factory reset.
Everything was working fine and then all of a sudden, I kept getting that error and my backup password wouldn't work. I restarted and turn off phone....even left it off for 2 hrs b/c I had to jump on a plane. I researched and found out 2 possible solutions...using Android Device Manager or Samsung's Find My Phone to lock the screen and set a new pwd. ADM didn't work (stating the fingerprint pwd was already set and to use it) and my Sprint S6 doesn't have options to setup Find My Phone. I called Sprint and Samsung support....no other solutions but to wipe my phone.
Now I have my bluetooth headset as a trusted device with Smart Lock since I can still access the notification bar when locked. Hopefully this will save me in the future.