Thank you, Robert, for reminding me about option 7. I had forgotten.
Is it possible that a mirrored pair of boot devices could get corrupted? I have scrubbed the boot devices, and the ZPool reports indicated zero errors.
Anyway, in desperation I tried a lot of passwords and got one to work, my actual password but capitalized. The system now requires that I enter my password in uppercase even though it was entered in lowercase. Once logged in I changed my password and made sure it was entered completely in lowercase. However, my system requires it in uppercase to successfully login. Seems incredible, but I have confirmed this multiple times now.