I just installed the new firmware and I just installed Plex media server for qnap "ARM".
until yesterday I had the Plex media server on windows 7, and it ran all right.
My problem is that I can't view any video-files, the server give me this error "An error occurred attempting to play this video."
Error in Italian is "Sì è verificato un errore nel tentativo di riprodurre questo video."
My files are .avi,.mkv, .iso, and noboby are possible to see.
I try to see, my files directly by browser (firefox) of my pc, and yesterday. before I installed Plex on my Qnap, I had plex media server on my pc, and I saw all file.
I think that it is not a problem of codec.
My samsung hubs give an error that it don't found files, all file it don't found, and Yesterday with my old configuration, it worked.
It may be that your NAS does not have the power to transcode the media, try installing the desktop client on your PC and see if you can see your files and play them.