Win NT4.0 wkstn Shutdown problem

In a Novel server environment running Win NT 4.0 clients with a novel client version 4.71 installed, I have a problem when I go to shutdown the workstation the screen goes grey and freezes. To get out of this situation I have to hit CTRL ALT DEL and it prompts me to unlock the workstation. Then I am able to shutdown without further problem.
What could cause this problem?
How to fix?