Just a suggestion, not a fix: If these are email accounts no longer used, I suggest exiting TB and using Windows FIle explorer to copy the message folders to Mail\Local Folders to keep them, allowing you to then remove the old accounts.

Thanks for the suggestion. I'd still prefer a fix for the bug. Until Tuseday it worked well. In the meantime I found a way to reduce the nuissance by changing the authentication method to one incompatible with the server. Does anyone have an idea?

In "Server Settings" none of the "check for new messages" is marked.

From memory, every time you open a folder for an email account Thunderbird will automatically contact the server for updates regardless of the "Check for new messages" options as they are only applicable for when Thunderbird starts or every so minutes. As David suggested, it would be best to copy the emails to the Local Folders section and remove the old email account. You can make new folders just like normal if you need to keep your emails organised.

The problem has disappeared by itself, without any deliberate change by myself, not even a restart.