VISTA RDP TO WINDOWS SERVER 2000

WE JUST SET UP A WINDOWS SERVER 2000 (PASADENA, CA) AND HAVE TWO USERS THAT REMOTE DESKTOP FROM DENVER, CO. THEY CAN CONNECT AND AUTHENTICATE BUT EVERY ONCE IN A WHILE, THEY GET AN ERROR MESSAGE THAT SAYS THE FOLLOWING:

ERROR (LOGIN): THE NETWORK REQUEST IS NOT SUPPORTED. (50)

I HAVE INSTALLED SERVICE PACK 4 AND UPDATE ROLLUP 1 AS ‘SUPPLEMENTAL’ HELP BUT USERS ARE STILL BEING BLOCKED.

CAN ANYONE HELP? THANKS,