No, that message is perfectly normal as the server expects a more specific HTTP request of your vSphere Client instead of a simple browser GET request.
I'm not sure what could be causing this error then, it sure sounds like it's simply not able to connect.
Just a guess but you could check these:
In the vCenter Server Settings -> Runtime Settings, make sure the correct FQDN and IP are set.
In the vCenter Server Settings -> Advanced Settings, check that the parameters VirtualCenter.InstanceName and VirtualCenter.FQDN match. Also take a look at parameters like VirtualCenter.VimWebServicesUrl.
You might have to re-register the inventory service in SSO too:
VMware KB: Repointing and reregistering VMware vCenter Server 5.1.x and components
