While I was logged in into the frontend and tried to access the Contao’s Backend it happend that my Browser told me it couldn’t load the page. A quick check with Firebug did show that Contao does some weird redirecting / sends 303 See Other all over the way. So I asked on the german IRC channel.
According to one guy on the channel this seems to happen occasionally and it is not limited to being logged into the frontend. I had no luck in finding a reason for this, yet.
It seems to have to do with cookies and/or sessions, though. Closing the browser did not help to make it disappear. Adding session_destroy(); did the trick.