I seem to have found the issue. Unfortunately it only seems as if the graphics card driver is the problem. It looks as if it is "x11vnc" in combination with xserver 1.9.0.
There is a thread to solve the problem here: http://ubuntu-ky.ubuntuforums.org/sh...1612704&page=3
The reason why it looks like an Nvidia problem is, because the is no crash report. And the "NVRM: os_schedule: Attempted to yield the CPU while in atomic[...]" message is always printed after a start of the Xserver. And since it ends up in login screen - X11 doesn't get startet then any more - it is the last entry in the log.
To make it final I will test this now. But it looks good so far.