Are you running a local web server, or is any other application running on your computer already using localhost? Anti-virus/security software tend do this.

Thanks Chris for your response. I 'm running standard Windows 11 and am not aware of a local web server. I am using Norton anti-virus. I have tried switching this off and restarting Thunderbird, it makes no difference. I also tried restarting windows in safe mode, this did not seem to help either but I am going to try this again.

Can you post a screenshot of the warning you get? https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

I am using Norton anti-virus. I have tried switching this off and restarting Thunderbird, it makes no difference.

Try to start Windows 10 in safe mode with networking enabled.

Still in Windows safe mode, start Thunderbird in safe mode. Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Here's the warning message. I'm not sure why it starts with the calendar. Nothing different from what is mentioned above. After several of these for each account I get a second message which I'll try to pick up.

As I mentioned I am using Windows 11. Probably you mean safemode for Windows 11? I'll trry that again later. Thanks

Thanks Chris, I think it is solved now. I started in safe mode as described. When I started up Thunderbird it went through the process of logging into the different email accounts - even though I had deleted the saved passwords it found them from somewhere. And now, back in normal windows operation it starts OK. Miraculous!