Hi,
I just got the the same problem on vcenter 5.1.
I did some research and followed the guidelines of someone who got the same problem.
Below are the steps:
1. Start Task Manager in the Local Windows Account(where vsphere client has been installed)
2. In the "Processes Tab", search for the "vmware-vmrc.exe".
3. kill this process
4. Log out of vphere client and log in again.
This fixed the problem
hope this can help you