Unless you are funnelling gmail through a local server (via apache or whatever) then hopefully https://support.mozilla.org/en-US/kb/automatic-conversion-google-mail-accounts-oauth20 should be sufficient instructions (it's referened from https://www.thunderbird.net/en-US/thunderbird/91.8.0/releasenotes/ )

None of that works. There is no local server etc. TB (64 bit) is running on WIndows7 (64 bit) PC. The email accounts are normal Google Mail accounts. One of the Google Accounts also has the Google Calendar. TB has been working for years but the update to 91.8 has stopped TB connecting to the email accounts but the calendar still works.

Could it be something with the Google Account settings?

have you checked the cookies setting as the linked article states? Have you tried clearing the cache in Thunderbird preferences? Given you are on windows 7 you probably have an anti virus with heightened security. Try restarting windows in safe mode with networking.

Is the authorization method set to oauth? I should be.

The cookie setting is Always Accept. TB Cache has been cleared. Anti virus is windows standard and there is no difference with it on or off. Same with WIndows firewall. The account authorization method is Oauth2. Just going to try Windows safe mode...

TB won't even start in Windows safe mode with networking. :(

I did a brand new install of TB 91.8 on a different Windows PC then added the Google email account (IMAP). It worked so it looks like the problem is with the PC and not the Google account.

Any chance it could be a corrupted TB profile? If I use a brand new TB installation / profile, the google accounts work. If I use the existing TB profile, google mail can't authenticate and it is not possible to add google email accounts. However, google calendar provider works OK.

I removed TB completely from the PC. Installed a clean version of TB 91.8 64 bit and still it would not set up a gmail account. I deleted TB again and installed TB v78.12 64 bit It worked first time.

Solution found :) O/S Windows 7 64 bit, TB 91.8 64 bit 1) Uninstall Thunderbird completely 2) Remove TB data files 3) Install older version of TB (64 bit) [DONT UPGRADE] 4) Add Google Mail accounts (and Google Calendar) in old version 5) Upgrade to TB 91.8

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Re installing TB 91.8 at step 3 didn't work, even with a fresh install and new profile. The only way I got it to work was by going back to an older version, getting that to set up the google stuff, then doing the update to TB 91.8