Im going to add to Pirahnas comment here, I have a Dlink router and its as new as can be and it does support UPNP but it wont work with the linkstation live. So what I had to do is turn off upnp and just use port 9000 like he said. I see you have done this already but try to turn off the upnp on your router. Also what kidn of device are you using to access your nas? (Cell Phone Web Browser, Laptop?) Is there a chance that you might be on a public network like say a coffee house or library and your router doesnt let you in because of that? I had to create a user name just for to be able to access my nas using the webserver porgram. All my regular usernames would not work for some reason. Also what restrictions do you have on the folders themselves? the fact that you can access it from your home network is leading me to think that is a router set up problem.
I realize alot fo these are shots in the dark but i'm hoping something will click. Also if you find the answer post it back here, I would like to know the solution and im sure if someone looks at this later for an answer to their problem, they might want to know how you resolved it.