Do you have js and/or cookies disabled?

After trying so many things, including rolling back to earlier versions, booting into safe mode, all I managed to do was invoke the same problem with gmail accounts as well. So after 10 years or so, looks like I'll have to look elsewhere. What a bummer.

Beyond allowing cookies and Javascript:

Make sure you do not use a VPN.

Turn off your anti-virus software during the authentication attempt.

In general it is recommended to create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-do-i-find-my-profile

Are you running some sort of software like a web server, which is blocking TCP port 80 on localhost? This can also be anti-virus/security software blocking that port.

> After trying so many things,

There are many possibilities. Not knowing what you tried, the shortest path to eliminating many of those possibilities is...

• Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode
• Still In Windows safe mode, start Thunderbird in Troubleshoot Mode - https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Please post your results.

Double safe mode, no good. This is a relatively new Win11 install, TB was all setup and working initially, so what has changed is the conundrum.

It seems to me the internal (?) browser is failing to connect, no cookies are set, nothing at all happens - outlook/gmail/yahoo, none of them work. Pasting the attempted URL into a real browser works, but it then tries to send a token to localhost, which fails.

If you use something like Apache Xampp to manage a website, it uses the localhost so you would need to switch it off until Oauth is up and running.

You may also need to access Firewall which may be controlled by the computer default or by an Anti-Virus program you installed. Make sure Thunderbird is set up as an allowed program.

Well, I gave up trying to figure what went wrong, but luckily found a workaround. DId a fresh install of Windows in VirtualBox, set up all my TB accounts again, then copied the profile back on to my main machine, and lawks-a-lordy, it worked!

Phew, and thanks to all for trying to help.