I got the Same problem....
I have a Folder Shared and that same folder is also set has the root for the uPnP Media Server.
the DNS-323 uPnP Media Server works fine when I use my Xbox 360... I can play any AVI file I have.
And with the SAMBA shares I can any file from my DNS-323 to a WD TV Live... even MKV videos at 1080p.
Also tried via Media Server... and it also works fine.
But If I use Windows Media Player 12 that came with Windows 7 64 bits,
I can play any file via the SAMBA share, but if I go into the WMP Library,
the DNS-323 does apear under "Other Libraries", but if I try to open the "video" Folder
I get the same error msg as chemi:
"The remote media library did not allow the connection."
"Please contact the device manufacturer for more information."
I also have a Linksys WRT610N, with a 16GB USB key, and I activated the uPnP on it... and
it works fine In WMP, Xbox, and WDTVLive
Edit:
Forgot to mention that I am using Firmware v1.08b8