What could be causing error: "Receiving NAT punchthrough attempt from target ############ failed"

I'm attempting to make a multiplayer connection from a friends computer to mine,

• begin hosting a server using NAT, I have the port I'm using forwarded on my router

• have my firewall off

• have my anti-virus disabled

Still, just as my friend tried to connect using my GUID This error apears on my Unity Console:

 Receiving NAT punchthrough attempt from target ################### failed

where ############### is the GUID of my friends machine.

What causes this error? What factors could potentially be causing the connection to fail like this?