I'm going to suggest something based on what someone with a similar problem reported worked for him, without me really understanding why it might work… :)

It looks as if Thunderbird was silently failing to authenticate with the server, without reporting an error message… And it seems that password related problems can sometimes be fixed by removing the password from Thunderbird, even if it is correct, just to make Thunderbird save the password again…

So make sure first that the account is properly set up to authenticate using OAuth2, as described in the following article: https://support.mozilla.org/kb/automatic-conversion-google-mail-accounts-oauth20

Then proceed as follows:

1. Go to Settings > Privacy & Security > Saved Passwords.
2. Look for 'imap', 'smtp' and 'oauth' password entries associated with the account that has the problem, select them and click Remove.
3. Restart Thunderbird.

Please, post back to report what happens if you try this.

Thank you for the assistance DavidGG. An extraordinary clear and concise walkthrough!

I went through each step to ensure I had the appropriate settings, and the OAuth2 etc., were all set properly prior to this. Glad I confirmed that, though.

The final check for proper authentication tokens appeared to be consistent with the article; however, upon extending the dialogue box to see the full details, there was further info that was unique only to the "failing" secondary email:

oauth://accounts.google.com (https://mail.google.com/ https://www.googleapis.com/auth/carddav https://www.googleapis.com/auth/calendar)

Not sure why this oddity is there, and if it may be the culprit?

No, that's correct. Each of the items within parenthesis refers to one of the scopes the OAuth token can be used for (mail, contacts, calendar). You may have that split into more than one OAuth token, but taken together, it should correspond to the things you may see you've granted access to if you check the Security section of your Google account.

I removed all the authentication tokens for the failing email address: oauth, imap, and smtp, and then restarted Thunderbird. That made Thunderbird (and ME!) very happy and brought back the account!

Thank you so much for all your efforts DavidGG! Greatly appreciated!

Sue

Great! And thanks for the feedback. Not that I understand why, but now I have confirmation that the suggested procedure can be used to fix this strange problem… :)