Few thoughts from my end-
Is your network drive mapped with a drive letter (IE. Drive D:\), or do you browse to it through a standard network shortcut. Either way, try the opposite
Have you tried switching the video renderer in MC's options? I'd suggest trying it with the legacy renderer, which has fixed a couple of odd Real Media issues for me.
I'd also mux the file into a proper as opposed to renamed MKV & see what happens. (Pretty certain it'll play with no issues, but thats not really the point here!)
My final thought, would be to try the latest nightly of ffdshow-tryouts, again this may be a funny bug from their end (This isn't a route I'd really advise you to follow TBH- I have a very bad beta habit, and these can and will break things; If you're not sure, don't)
-Leezer-