It only happens when I start compiz with X (from a script that is called by xinit/startx/whatever calls the script). If I start X and then start compiz (over ssh or from a gnome-terminal) then it works fine. 

If started with X compiz will crash once a window is opened (when I start gnome-terminal or glxgears)

I have attached the crash output from compiz (thanks to crashhandler plugin). Here is the x part:

#4  0x00007f261a5c4db9 in memcpy () from /lib/libc.so.6
#5  0x00007f261b5b1129 in miRegionCopy (dstrgn=0xc61ff0, rgn=0x7fff26427e40)
    at Region.c:553
No locals.
#6  0x00007f261b5b2258 in XUnionRegion (reg1=0xc61ff0, reg2=0x7fff26427e40, 
    newReg=0xc61ff0) at Region.c:1297
No locals.


I had a look at the code, and put some fprintf's to try figure out why it was failing. It seems that miRegionCopy would break if destrgn->rects was NULL initially (as memory is only (re)allocated if destrgn->rects != NULL) - however, after patching this (potential) problem, it was still crashing. The debug output indicated that destrgn->rects, rgn->rects were both not NULL, and numRects was 1.

Is there any more information I can/should give?

Thanks for your attention, 
Joel.