That error seems to happen frequently. Somewhere in your profile, you've had this before. Thunderbird has it as a server somewhere. If you search everywhere and cannot find it, then I suggest you exit Thunderbird, copy your prefs.js file for safety and then use a text editor, such as notepad, to see if there are leftovers there. Usually, this happens when you attempt to add another account that uses same domain, such as multiple gmail accounts.