Are you using OAuth2 as the authentication method?

Yes! And SSL/TLS for Connection security Any other ideas? Thanks!

Run Windows in safe mode to bypass startup apps like ESET. Does TB then connect?

Thanks #sfhowes. This is a relatively new computer for me, and safe mode doesn't look as trivial with Windows 10; a BitLocker key may be needed, and I don't know where mine is (if I have one). So, I disabled ESET instead, which didn't make a difference.

The problem must be with TB 78, as everything worked perfectly until the auto-update... and only TB has a problem now. I'm accessing my mail via gmail in Firefox and on my phone just fine. There must be zillions of people in the same boat!

I don't have any problems with a gmail account in TB 78, and I'm sure that's the case with millions of other users, otherwise there would be a deluge of complaints. It often happens that when TB updates, the installed antivirus becomes incompatible. If you don't run Windows in safe mode or remove ESET, you should at least add the TB profile folder as an exclusion and probably disable SSL filtering.

You're a wizard #sfhowes! Disabling and then re-enabling SSL filtering did the trick, and 350 e-mails dumped after a reboot. I very much appreciate your persistence and expertise! I really like TB, but wish this major update was associated with at least a warning that there may be issues. ESET is a fantastic product, so I can easily imagine worse problems with other antivirus applications. Anyways, thanks again!