How do you check the password, since it needs to be Oauth2? It can't be the same as your web password. A possibility is that the accounts may be crossed. Check that you have two incoming servers defined and two SMTP servers defined and that the SMTP servers are each assigned to the proper account. That may not be the issue, but I've seen similar issues that were.

Thanks David To check my password I use Tools>settings>Privacy and security>saved logins. That shows the correct password next to smtp://smtp.googlemail.com (smtp://smtp.googlemail.com). Authentication is set to OAuth2. What is strange is that all my other accounts have comparable settings etc yet have no problems.

OAuth2 with gmail on TB has problems if you're using the googlemail names. Change to smtp.gmail.com (and imap.gmail.com or pop.gmail.com). Remove the passwords and oauth tokens from Saved Passwords, restart TB, enter the account passwords in the OAuth window.

Did all that without success. Is there a way to delete the account yet keep the data? Maybe recreating the account will work

If the account is IMAP, mail is stored on the server, so the account can be removed and re-added without losing anything. For POP, copy the existing mail to Local Folders before removing the account.

I did use Imap. I deleted the account and recreated it. Problem solved. Thanks