Hi, what is the UPNP media server software? I have noticed that the UPNP discovery of servers from the SC200 is limited, but it should recognise any UPNP server at startup - I would check you firewall settings and ensure the UPNP side is not restricted (try diabling it and test)
I have used Twonky with no problems with my SC200. (You can download a 30 day free trial on your nas a test it - your nas is not natively supported but it may be worth a go..?)
http://www.twonkyvision.de/Products/TwonkyMedia/nas.html#pre-installed