No logon screen after Ctrl-Alt-Del on Windows 2008 R2 TS

Hi,

We have got a problem on our 2008 R2 TS servers that the logon dialog is not showing after Ctrl-Alt-Del is pressed.
The screen is just blank and nothing happens. Even on console.
The Novell client is Novell Client 2 SP1 IR9 and ZCM 10.3.3.
After a reboot it is working for a few hours, and when back to no logon screen again..
Have tried to reinstall Novell client, did'nt help. The machine is fully patched with Windows update.

Any ideas?
Please help!