Sometimes when I try to log out, I get this error message and log-out fails. MH is the only site I experience this problem. I'm using Firefox 8.0.
It's not a big deal, I can delete the MH cookie from Options -> Privacy -> Show Cookies, it's just a bit inconvenient.Though if you want to log out and it won't load like that, you can simply clear your browser's history and cache or something like that; it usually automatically logs you off from all sites you're in currently. ^^