Problem: The request refers to an object that no longer exists or has never existed.
Solution: This would happen in different scenarios. Restarting VirtualCenter Agent service in the host would most likely fix the problem (command is "service mgmt-vmware restart). If not, restart the VirtualCenter service in the VirtualCenter Server.
P.S. The platform here is ESX 3.0.1 and VC 2.0.1.