I've had the same setup of my gmail accounts for years in Thunderbird, but today one of them stopped working. Whenever I try to get or send mail, I get the pop-up OAuth window for logging into Google. I do so successfully and select "allow" to give access to Thunderbird. But then, back in Thunderbird, I see "Host contacted, sending login information..." and it never finishes. Eventually (many minutes) it times out. I have tried dozens of times with the same result. I have a second gmail account configured in the same instance of Thunderbird, with exactly the same settings. It continues to work fine, and does not display the OAuth pop-up when I get/send mail. This makes me think that the problem has something to do with my Google account rather than with my Thunderbird settings, but I have not changed anything there either. Both accounts are configured for pop3 (pop.gmail.com:995), security SSL/TLS, authentication OAuth2. Thunderbird is set to accept all cookies and keep until expired. I deleted all saved passwords for the failing account, but Thunderbird still has the password for the account that is working. I've already wasted many hours on this. I have read and tried all suggestions that I can find online. Please help!