User Logged on to too many workstations

I’m working with a network running Novell 4.1 and trying to solve this error. It seems to creep around to different users with no pattern. When a user logs onto thier workstation, wether they are running Windows 95, 98 or NT, they receive an errorthat says “User is logged into too many workstations simultaneously” and they are not. Thanks for your help.