Downgrading xorg-server to 1.12.4 fixes this issue.
I will create a backtrace as soon I find the time.
No need to produce a backtrace - attached upstream bugreport, which is already [resolved fixed].
Created attachment 331298 [details]
Upstream patch, applies fine to xorg-server-1.13.0-r1, and (more important) fixes the issue with xrestop.
I can't reproduce this, but I am sure the patch should work fine. :)
xorg-server-1.13.1 is out, which should carry this fix (at least the ChangeLog mentions the commit ;))
1.13.1 is in the tree, so marking as fixed.