Created attachment 618571 [details]
Xorg.log from the crash

Since the youngest update of xorg-x11-server (done on 2014-12-28), we're seeing
crashes of Xorg occasionally (~ once every second day).

Each crash started with the following situation:
Two sessions (on Ctrl-Alt-F7/8) are running; in one session (F8) there is
VirtualBox (QT) running.  When the user switches from F8 to F7 and back to F8,
the session on F8 crashes.  The user suspects that the F8-server crashed when
switching to F7, but is not certain.

Attached you find the Xorg.log file which was created before and during the
crash.

So far we're not able to reproduce the crash reliably.

A side note: The machine has an nVidia GeForce4 MX 440.  However, due to Bug
851394, the machine uses the "modesetting" driver.