At least at first glance I would say this implies that it is attempting to communicate with a vCenter that is no longer responding. When you say you upgraded vCenter, did you do an in-place upgrade, or install a new vCenter and migrate hosts? If the later, is there any chance that the old vCenter never removed from the Veeam console? The other common reason I've seen is hosts that are offline, etc. Some additional details of the failure would certainly be useful. Are you running Find-VBRViEntity with a -Server parameter specified?