The issue is an AVG issue, not a Thunderbird issue. AVG is obviously interfering with Thunderbird execution. The solution is to either remove AVG or investigate what AVG is doing. Many antivirus programs have options to bypass email clients. You might also contact AVG for an answer. I will also share that I use Win 11 and have used the builtin Windows antivirus for several years and it works quite well.

On mine new computer with Windows 11 Thunderbird is not working when AVG free is installed. Is this an known problem?

AVG is a known offender. See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVG

AVG is scanning encrypted connections and replacing legitimate website certificates with their own. This is also called a 'man in the middle' attack. See Also see https://support.mozilla.org/en-US/kb/error-codes-secure-websites#w_avastavg

It works well if I uninstall AVG free.

That would be reason enough not to use it. I'd just use Windows Defender, no need for 3rd-party anti-virus solutions.

As a general recommendation to avoid problems with anti-virus software, create an exception for the Thunderbird profile folder in your anti-virus software, so that it will not be scanned by the real-time scanner.

To locate the Thunderbird profile folder see https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Thanks for your reply, it still doesn't work, so I have deceided after many years to say goodbye tot AGV FREE.