You are failing to connect to Google with a 400 error. Nothing else you have written is even relevant really as it is discussion about a google web page served to Thunderbird as a web browser.

The relevant thing is the actual error and you have not posted it, nor I suspect the complete error message. I google error 400 and get results for

Error 400: invalid_request error 400 redirect_uri_mismatch Then there is 400. That’s an error. Your client has issued a malformed or illegal request. That’s all we know. Which you get I have no idea.

Could you please post the full error, vebatim please. Perhaps capture an image of the page.

Have you checked your settings at Google to enure you have not disabled or disallowed Thunderbird's access? Have you tried deleting any passwords for the account, especially for the oauth:// type entries? https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb Have you reviewed the troubleshooting information here pertaining to passwords, JavaScript and cookies required for oauth to work?

It seems to be related to my installation of Malwarebytes. I have used it in the past without a problem, but this error manifested after I reinstalled it, though there was no message from Malwarebytes that it considered Thunderbird or Google to be suspect I did a system restore to before MB installation and the problem went away.

So malwarebytes was doing the usual antivirus thing and messing up connectivity. It has become far to common in recent years in my opinion with all of the Third party antivirus products. Most of the security software appears to work correctly only if you do nothing on your computer but browse the internet and get mail using a web mail site.

I have conceded and only use Microsoft Defender after a number of years with ESET as defender actually appears to be designed to work with software other than just a browser. It is not as good at holding your hand, and it is not prone to wildly inflammatory statements and flashing warnings of risk like the third party products. But I do appear to be virus free and that is what I want from my malware protection. Not a free VPN, not an encryption breaking scanner, not a trust model that says I trust the antivirus vendor implicitly. I just want to be uninfected by malware.