What are you doing right before you receive the error?
Have you read the following KB: 1035639
This is a common vSphere client error that can stem from a number of things. Typically it's just a matter of checking your connection and trying again. I've noticed it from accidentally issuing the same command twice while the first is in progress, etc.
As mentioned above, whatever you were doing before you got the error would help in diagnosis.
check if hostname is registred with FQND in DNS. And check firewall. http://kb.vmware.com/kb/1012382
good luck