You may see following problem if you login vCenter  Server 6.0 by vSphere Client:

Login to the query service failed.

The server could not interpret the communication from the client. (The remote server returned an error: (500) Internal Server Error.)

That’s because “Use Windows session credentials” checkbox is selected. Deselect it and give it a try.

Refer KB Searching the Inventory with the vSphere Client fails (2143566)