[TYPO3] RealURL works, but 404 when logged in at BE .. why?

[Christoph Luehr]

Hi,

i've set up a site with RealURL (its not my first) and the FE works
flawlessy, if an ordinary user accesses the Frontend.

As soon as a Backend User loggs in, he can't access
the same FE URLs anymore.

The URLs stay the same; the Apache logs register a 404
for BE-logged-in users and a 200 for ordinary users for
_the_same_ Frontend URL.

Any ideas? I would be very grateful for any hints, pointers, whatever.

Kind regards, Christoph Luehr