Seems like the NVIDIA-driver is causing my problem with the X-Server, on my notebook with an ATI card there is no problem. After switching from the proprietary NVIDIA driver to the nouveau-driver X doesn't crash anymore, but the canvas doesn't seem to update. Attached is a screenshot to show what it looks like. My system is running with the latest Kernel, 3.10.rc2 since today, before it was 3.9.2