The easiest way to enable UPnP is to go to
Unplug'n'Pray and save the app to your desktop. The you can click it and enable or disable UPnP whenever you want/need to. It may take a reboot and a little time for your router and/or WLM to recognize it's enabled so be patient.
I've had the loading problem where WLM will crash after some amount of time when it can't connect. It seems to happen with certain contacts, while others work just fine.
Making sure UPnP is enabled on both ends is probably the best thing you can do if everything else seems set up properly.
Let me know if UPnP solves the problem!