Quite often its something you cannot control, though in a weird twist, often if you restart the router it'll be fine. I've never got to the bottom of it either, I have an old D-Link router at home that occasionally goes "Strict". I restart it, and its fine for the next six months. Very odd.

well i dunno if this helps but when i changed the parental settings on my xbox my nat stayed at open. i just set the xbox to accept everything eg no filtering of content etc