Is settings clear cookies and try it again. My understanding is that Yahoo reuse cookies when setting up oAuth authentication. So removing the cookies between tries will perhaps stop them doing that and allow success.

Thank you for the advice. Unfortunately, even after removing all cookies related to Yahoo in Thunderbird, the problem remains. Would gladly welcome any other advice / suggestions.

I have just read this https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo#w_configure-a-thunderbird-hidden-preference-to-make-multiple-accounts-work

More details about hidden preferences at: Config Editor Click ≡ > Settings > General. Scroll down to the bottom and click Config Editor.... Click Accept the risk and continue if that appears. Type mailnews.oauth.usePrivateBrowser into the search box. Click on the Toggle Config Editor Toggle button to change the value to true. Quit Thunderbird (Control Q on Linux and Windows, Command Q on macOS) and then restart it. Retry logging in to your accounts (Yahoo, ATT, AOL).

Perhaps that is the answer.

Thank you again for the detailed advice. Had already tried this option as the same was listed in the Mozilla Support page. However, this has not helped. Glad to get any other advice / suggestions.