I've had the same issue.
If I attempt to play media on the DNS-321, via the mapped network drive, I get an error using windows media player. I don't get an error if I go through network places, to the same file.
On - another issue:
Also, when I attempt to play ripped avi files, w/ multi-channel audio (more than two channels, ie a ripped video w/ 5.1 audio as opposed to 2.0 audio) on my XBOX 360, I get an error code. If I attempt to play the same file through a DSM-520 it seems to work ok.
I will follow this post up with an error code # later this evening.
I'll followup this port