Windows 2000 Server Maximum Concurrent Logins

I set up a Windows 2000 server with 10CALs on the license in a VPN-accessible scenario. We have not had 10 different clients connect to the envrionment yet, however when more than two users with admin rights try and connect to the server they get the following message:

“The system cannot log you on.
The system has reached its licensed login limit.”

Each user is connecting via VPN and then using remote desktop to access the server. Can anyone help explain this issue and offer some suggestions to enable more concurrent sessions? Thanks very much in advance for any thoughts.