I suggest starting over. enter gmail email address and see if that is accepted. This particular feature has been rock solid for years, so I'm puzzled

Disable the account hub in Settings/General, start again from ⩸ - New Account - New Address Book, CardDAV Address Book, enter the gmail address.... Contacts are autodetected. Cookies must be accepted in TB (not browser) Settings for OAuth authentication to complete.

I suggest starting over. enter gmail email address and see if that is accepted. This particular feature has been rock solid for years, so I'm puzzled

Ok. Thank you. Do I delete that one account in Thunderbird then add it back in fresh? Should I turn off the "Account Hub settings in the image attached? (Since the normal way has been rock solid for years.)

Disable the account hub in Settings/General, start again from ⩸ - New Account - New Address Book, CardDAV Address Book, enter the gmail address.... Contacts are autodetected. Cookies must be accepted in TB (browser) Settings for OAuth authentication to complete.

Thank you. I think the 'account hub' and 'cookies' could have been issues for me.

Solved. Thank you both. I had also manually remove the 'contact book' after removing the account, but before adding it back. That allowed the set up process to add the book as a new CardDAV book. Looks like it is all working now. Thanks!