Adding to original image what happens when you select 'Next'.

That's the standard OAuth2 authentication window for gmail accounts. Check that cookies are accepted in TB Settings/Privacy & Security, remove any entries for the account in Saved Passwords, restart TB, and see if the process completes when you enter the account password in the window. A VPN or some antivirus apps like AVG/Avast could also be blocking.

I think this might have solved the problem. Just waiting to see if the log in screen continues to pop-up. Thank you.