Any update on this issue?
I don't think there are any updates to the UPnP server coming for the DNS-321. Unfortunate, but the DNS-321 is pretty much orphaned by DLink.
If you have problems with the UPnP server, it may be better to root the box and run the UPnP server that is available in the fun_plug tools.
The DNS-321 UPnP server is not quite DLNA-compliant, and has lots of access problems from DLNA clients. It didn't find any files when accessing it through my BlueRay player even though I could navigate the directory structure, for example.
When I started running the Mediatomb UPnP server that comes with fun_plug, everything works fine.
Of course, you may need some technical knowledge to actually root the NAS. Search for "fun_plug".