FacSys causes Spooler to stop in NT 4.0

We are running a program called FacSys that allows our users to send faxes directly from the desktop by printing to the FacSys server.
It usually works fine, but I have one user using an IBM NetVista tower with NT 4.0 that is having problems faxingfrom secure sites on the internet. Whenever he tries to print to the server, it stops his spooler service, and when we restart it, it says that the user cannot be authenticated to the Fax server. His username & password are correct. Does anyone have any ideas? Desperate to solve this, he is a securities trader, and this is an important function for them. Thanks!