I think I know the problem.. I get that error often, after I have loged in at other sites with an other password, after I have pressed "remember me" at TWHL..
So when you login on an other site, and it creates a session or a cookie.. it over writes the TWHL cookie / session (what ever TWHL use)
and changes the password, and when you try to login on TWHL with that cookie / session, the password / username is wrong = invaild cookie..
[/Madcow's theory]