GDM crashes when disabling the internal screen

[C_Classic]

Binary package hint: gdm

I have an external monitor (Acer T231H via HDMI) connected to my notebook (intel graphics card). Cloneview, dual view and only internal monitor works fine.
But when i disable the internal screen (-> only external) both screens turn off. After 30 seconds both screens turn on again but only the mouse is visible.
Restarting GDM using a terminal (Ctrl+Alt+F1) brings the interface back in Cloneview.

PS: When I keep on moving the mouse after I clicked <Apply> it works...

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: gdm 2.32.1-0ubuntu3
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
Architecture: i386
Date: Fri Apr 29 12:11:57 2011
InstallationMedia: Ubuntu 10.04.1 LTS "Lucid Lynx" - Release i386 (20100816.1)
ProcEnviron:
 LANGUAGE=de_DE:en
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: gdm
UpgradeStatus: Upgraded to natty on 2011-04-28 (0 days ago)