Bug#640255: Xorg does make black screen and crash on exit

["zemir_FP@libero.it" <zemir_FP@libero.it>]

Package: xserver-xorg-core
Version: 2:1.7.7-13

I have installed backbox windows manager, when i start xserver xorg all does 
work properly but when i exit, the screen does make black and the sistem is not 
usable. I try to change session with "ctrl-alt-del" but i see anything; i try 
to kill xserver with "ctrl-alt-space" but i see anythig, too. The only way is 
to make "ctrl-alt-del" and to reboot the system. I am not able to start "sudo 
xorg -configure" because i see a black screen and the only way also in this 
case is to reboot the sistem.
I have a small laptop "panasonic cf-m34" with a silicom motion video chip 
controller and 4mb video ram.

I am using debian squeeze 6.0.2 with a generic 2.6-i686 kernel.