It seems that I have the same problem. After upgrading to Lenny, my X server often crashes, then it seems that it wants to restart it, but without success, and finally I get an error message. I attached the Xorg.0.log to this mail. I am not sure if I have the same problem than Luca but the log file of the X server looks very similar to the log file attached by him.
Finally some informations -- probably that helps:
* I do not use Compiz,
* and the last three or for times the X server crashed while using
Open Office.
Best regards
Raphael
Attachment:
Xorg.0.log
Description: Binary data