No Inventory in WebClient (while vSphere Client is fine)

I am a VMWare Newbe.

I installed VMware vCenter Server Appliance 5.5.0b released. (followed the TinkerTry instructions Deploy VMware's vCenter Server Appliance 5.5 in a home lab, without SSO errors - YouTube)

I could not configure a DataCenter over the WebClient, so I switch to vSphereClient.

Now I can't see the DataCenter (and all the inventory) in the WebClient (vSphereClient is fine).

If I try to create a DataCenter over the WebClient I get the message that "datacenter" is already there.

What could be the problem?