On adding a couple of new devices behind my firebox firewall I recieved the most unhelpfull message "user count exceeded". I double checked everything and after many weeks of fighting with the issue I finally found the solution.
What the error message means is that there is a licensed limit (based on the key you've purchased) for the number of IP addresses on the local network that can use the firewall to access the external interface. The error message most un-helpfully calls these IP Addresses "users".
So where you have upgraded a number of machines, changed the IP addresses, and the odd printer has chatted to the internet all these old IP addresses are stored on the Firebox. These then block any new ones for new devices and have to be manually removed.
To do this open the Firebox System Manager ->Authentication Tab->IP Address button
This will give all the addresses that the Firebox has remembered for you as having access to the external interface.
Either clear them all to start a new list or remove the ones you don't want!
This link eventually told me what I needed to know.