No one? Tech Support seems stumped; after 3 hours on the phone, they said they needed to talk to the higher support backroom engineers. :-) I guess I am the only person having/has had this problem?
The Pre-Upgrade Host Agent check that comes with vCenter 5.1 passes flawlessly; it's this test that does not. I may just go ahead with the upgrade, if I don't hear back anything soon. I have a deadline to get this upgrade done.