I'm afraid there isn't enough information in your note to diagnose the problem. I had essentially this problem trying to run the 10.1.5 version of Server Monitor to monitor a 10.1.5 Xserve from a 10.2 client: Apple eventually confirmed that you can't use 10.1.5 Server Monitor from a 10.2 machine in this way, although 10.1.5 Server Admin works fine. Eventually I had to downgrade my desktop machine to 10.1.5 to be able to use Server Monitor (since Apple's OS X up-to-date program still hasn't sent me 10.2 for the Xserve that arrived the same day Jaguar came out....)