Usualy the "Not found" happens when vCO has a reference to an object but is not capable to resolve it.
It is odd it happens in the web view but not in the java client. If this was the case GSS would handle the case.
The fact that you moved to 5.1 still means you got the new vCenter plug-in using the query service. You may try to shut this service down on vCenter side, restart the vCO server to see if this solve this issue.
