kdm/xdm don't start WindowManager

[Reto <blackhole1@gmx.net>]

Hi!
I have Debian Sid installed with the current XFree86 and current drivers 
for my GeForce. The X Server runs fine, I can start kdm or xdm without any 
problems.
But when I start one of them and it asks me for login, it is not able to 
start the WindowManager itself. Screen is black, then it returns to the 
login screen again.
I tried with KDE2 and WindowMaker. But: I can start KDE as well as 
WindowMaker by hand! They work properly, everything's fine.
Why can't kdm or xdm start the WindowManager? X itself IS running...
  Tnx a lot
     Karl