I just wanted to add my 2 cents. We've recently just upgraded from vsphere 5.5 to latest 6.5. The same issue happened to us, veeam would not query the tags and failed in the same manner explained in this thread. For us our "VirtualCenter.FQDN" was set to just our hostname. So obviously after adding our FQDN, also rescanned vcenter in veeam console, it fixed the issue. Thanks for the help!