Hi,
Have you taken a look at http://geeksilver.wordpress.com/2012/12/04/vsphere-5-1-x-vsphere-web-client-cannot-connect-to-vsphere-server/ ? it may be the wrong FQDN was entered during the vCenter installation. The other thing is to check your DNS tables for that ip address to see if it provides a clue as to what that IP address's hostname was