Try running:

scenter -reportlic

from a command line in the RUN directory on the server.  That will tell you info about the license code.  Make sure your code has the right options in it, including number of users.

That sort of error sounds suspiciously like a bad auth code.

  Its possible the customer has purchased a 0 floating user license e.g. all their users are categorized as named users.

   When you run scenter -reportlic you'll see something like this:

Expiration date:  11/23/2002

Server Quiesced State    :  Allow All Logins

Licensed Client Platforms:  All

                 Licensed     Active
Named Users:          0            0
Floating Users:       4            1
Limited Users:                   0

   There are three categories of users:

    Named users are specific people who can use the product e.g. John Doe, and only John Doe can use John Doe's named license.
    ServiceCenter keeps a list of the people who haved named user licenses in something called the named user file (check your doc on this as they may have moved the data into the internal data store by now, I haven't looked since SC 3.0). If you're in the named user file, then you can used a named license. If not, no dice.
  
   Floating users are licenses that can be used by anybody who tries to log in. Thus if I have 10 floating licenses, any 10 people who have valid UID/Password combinations can log into the system.

   Named licenses, as they are more restricted, are cheaper.

   If your customer has a license code that doesn't allow any floating users, then you'll need to put yourself on the named user list in order to log in.

   Hope this helps.