Same on LInux v91.8.1, but only my gmail account failed amongst four different domains.

• Turned off all the server check on launch and timer preference switches for only the gmail account, so I could stop the scourge of login requests. Incoming mail confirms at least one of the other domains continues to work, and reboots behave normally with gmail checks disabled.
• Saved passwords with gmail had a random string I'd never seen before, so first, I tested and proved my password using their web interface, then reentered that password in saved passwords manually; no resolution.
• Deleted gmail entry completely from saved passwords, assuming it would repopulate when prompted by getting mail. It did not.
• Toggled server checks back on to get the routine login prompt, but it won't recognize my login with that TB web page popup. Cycles a screen refresh and asks for email again as if my address wasn't found; I never get past that screen to enter a password.
• Reminder link works, but entering my backup phone number only cycles to a generic google help page, of no help.

Something may come to me in my sleep but this is odd. If there was some kind of gmail IMAP mishap, that just happened to coincide with my TB update relaunch, then I should go buy a lottery ticket.

At this point, I don't know what else to check as the account and password work using the web interface, as they did an hour ago using TB, and the major change is the TB update. Things like ports and addresses appear unaltered. The only found oddity was the strange saved password.

Working now, after two reboots from crashing TB.

One reboot out of ideas, then a TB crash while browsing through account preferences forced a power cycle, and the second reboot came back and TB worked as expected, requesting my password with the very TB-looking dialogue when I tried to open the gmail account and not that google web page weirdness. My web interface password wasn't accepted, but worked with a 2nd attempt using the odd string I mentioned--and saved. My saved passwords gmail/IMAP entry was automatically recreated as expected, and I turned my check-on-launch switches back on, confirmed okay.

I'm still confused why my gmail password isn't the same as my IMAP password, but I'm good now.

I wonder if Oauth2 is failing?

I just realized I had forgotten to switch my authentication setting back, and it failed again on Oauth2. Leaving my gmail on "normal password" works. I read about authentication complaints elsewhere, and I'm pondering if Oauth2 has linked methods across the web that would affect multiple mail servers and have nothing to do with TB. All my other domains were set to normal password.

Alternatively, did something about the TB update switch the authentication method from normal password, even if it was just a corrupt bit or gamma particle or something.

Lastly, reading the original post, when things were failing my status never did say connected, just food for thought.