I encountered same issues recently, in my case, the IP of my web client PC (192.168.5.*) is not in the same subnet of my ESX hosts (192.168.1.*), also I could not ping 192.168.5.* from 192.168.1.*, but I could ping 192.168.1.* from 192.168.5.*. My ESX hosts all nested on a powerful Windows 7 desktop (5.*), so I setup PPTP VPN on this Windows 7 desktop, then VPN to it from my client PC, it solved all the problems. One critical thing of using vCenter is make sure you can ping each host, and if you use Active directory, each host need to be resolved from client PC!
Since you use administrator@vsphere.local, so role(permission) should not be the reason!