I know this is a very old thread but after researching my problem and ending up here I found my solution, which may help others.
I could make several downloads over a day or several days with my listening port open. Then it would become blocked so I would try resetting everything, rebooting router etc., after much messing about I would get the port open again but without knowing exactly what fixed it.
Re: post #2 - recheck the port by hovering over it and clicking the right button, then "check again".
There is another option - Retry UPN port mapping
And it worked for me, several times now. So hopefully this will help someone else in the same boat.