Probably a bit late to help you, but I ran into the same problem after adding a VMKernel NIC to the portgroup in question.
Despite the message, the VM that was apparently 'broken' was still able to use the network interface through that portgroup. I shut the VM down to increase its RAM, but noticed the web interface didn't report the increased size either. After I removed the VMKernel NIC from the group, the correct RAM size was reported, although as far as the GUI was concerned I needed to re-associate the portgroup with the Virtual NIC.
This type of bug doesn't seem very unusual from the javascript GUI - it's not the first time I've seen the properties of a VM get mangled and it almost always crashes and has to be reloaded when I power-on a VM.
For infrastructure that I'm relying on, this kind of thing makes me very nervous.