Having this same exact issue... any progress on getting it resolved? Or are you still unresolved?
Take a snapshot of the vCenter Server VM, then reset the Update Manager DB using the instructions in the following KB:
This should fix the error message you get when you select a VM and click on the Updates tab.
If anything goes wrong, revert to the snapshot that you took.
This usually happens when the VM hardware version is 15 and Update Manager 6.7.x only supports up to HW 14.
I would suggest you to check if the VM hardware version and if its 15, then we don't have any resolution at the moment.
Work around may be to downgrade the HW to 14.
vSphere Install-Upgrade Support