Readynas 104 gives errors during playback via VLC.

For the last days I have been using Windows Media Player (WMP) instead of VLC.
For the first 2 or 3 days playback was fine and I was thinking it could be a solution to use WMP, but then Yesterday I got an error reading something like this from WMP "Playback stopped due to an unknown error". Restarting playback works imediatly, and then the file plays without error.

Much the same problem I experience with VLC, just VLC's error message is different.
My conclusion is that the error is not only present with VLC.

But why does this error only show itself while I'm playing video's, and why can I copy files to and from the NAS without errors from the same computer?
Why is playback of the same files fine on my media center fine through NFS?

I'm really puzzled by this, also because I never saw this problem before updating to 6.2.

I'm actually sorry that I did, as in hindsight I would rather have stayed on 6.1.9, since that seemed to work without errors for me.