SBS Client Licenses

I have set up an SBS2000 for a client and added 5 CALs to the default 5. Even with only one or two PCs connected, the server is generating “out of licenses” errors from SMB connections and then locking client machines out. The server is running the usual SBS suite (SQL, Exchange etc) and BackupExec, along with usual file and print. Nothing out of the ordinary. Mix of W2kPro, NT4 and Win98 clients. Drives are mapped to shares. Anyone had this before?