Did you ever get a fix for this issue ?
I recently got a DNS-343 in order to offload some of my movies to, and I have the same issue, even when using XBMC (for Mdia Centre) under Windows 7 x64. DNS343 Server is seen, I can browse the UPnP server and see all the movies (some AVI [divx encoded], some WMV, and I even copied the Sample movie from Windows 7 to keep it simple) , but when I click on the movies, they fail to play.. However if I browse the box Via the Samba share, I can play them.
When I point XBMC to any of my other 2 NAS (Syn*l*gy DS508 and a QN@P TS409 Pro), it can play any of the media without an issue. Even a copy of the files on the DNS343 which I transfered over in case they had gotten corrupted.
I would have assumed XBMC as the problem, however it can play the same files perfectly well off the other 2 NAS units..

Any info would be much appreciated.
Thnx in advance.
BTW: I am still using firmware 1.03