As you sent a simple request and that doesnt work then for sure it sounds like a computer issue not an sdk issue as either the request isnt getting to the server which is an outgoing rule issue, or the reply isnt coming back. This could also be your router misrouting the requests back. Can you sniff any response at all that matches the reply you should be getting but do it local network wide incase uPnP is not quite doing its job correctly
Also maybe completley disable the firewall on the problem machine just until you finish testing. , make sure all AV is off, make sure you dont have a rogue install of an old firewall and/or antivirus software left installed somewhere