We're at the point where it might be a good idea to open a ticket.
First thing GTAC should do is take a trace on the NAC appliance and see if the RADIUS request is being received, and if it's being responded to. The initial error message indicates there is a failure to process the packet.
You can take a trace on the NAC using tcpdump, and you can enable diagnostics in the NAC webview if you are comfortable with it. I would suggest Authentication Request Processing - NAC and Authentication Request Processing - RADIUS.
If you aren't comfortable I suggest submitting a ticket and speaking with GTAC.