[TYPO3-debian] login screen when clicking user admin

[svrmarty]

hi,

only when i click on "user admin" in the backend, i get a login-screen.
all other options work. when i type in user/password, i get a new menü on 
the right side.
when i'm clicking two times on "user admin" i get
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.

same happens, when i'm calling the mods directly
this works
http://server/typo3/typo3/sysext/belog/mod/

here i'm getting the login screen
http://server/typo3/typo3/sysext/beuser/mod/


it's a debian etch machine with apache2 and php4 with  libapache2-mod-php5
first i tried typo 4.0.2 (etch package), then 4.1.5 (lenny package)
all debian standard


how could i track this problem ?