Additional features, settings, or issues not covered by specific Microsoft Teams categories
Update:
Used a 2nd Win 10 Pro PC to download Teams and try the guest @gmail.com login that failed on 1st PC. The Guest login worked just fine.
I removed Teams (and the Teams Machine-Wide Installer) and re-loaded Teams. on my 1st PC.. still could not get Teams to login and it fails again with 0xCAA20003. Clearly the Teams client on PC #1 is corrupted / registry (?) blocking login. This same PC works for my corporate login to Teams.
This is where people would say it works 50% of the time. Not an issue with the Guest account but an issue somehow on the Windows client PC doing the logins.