[TYPO3-english] Backend logout after viewing frontend page

[Alex Bailey]

Hey guys,

I'm having a rather odd problem with a typo3 installation (4.2.10) on my 
local server. When I log into the backend I can do anything just fine 
but If I visit any frontend page while being logged in I get kicked out 
of the backend. No mather how I view a frontend page (open it or viewing 
it in the backend) I get logged out with the following message:

Login-error or session timed-out

No user logged in! Sorry, I can't proceed then!

(You must have cookies enabled!)

If your session has just timed-out, you may
click here to re-login.

This happens on pages with and without extensions so I think it might be 
a Typo3 or Server Problem.