It might be that some character in the file name is not getting parsed correctly by either the Roku or when handed back to the ReadyMedia server.
Try renaming the file to something simple like x.mkv
You may also want to confirm you are using version 1.2.1 and also confirm that the ReadyMedia service has rights to read the file. If you continue to have problems, try checking for any logs produced by the ReadyMedia service.
I've said this here before, most problems involving dlna or media servers are not the fault of the roku player.
Glad its working. Stick to those few codec restrictions and you'll be good.
I got it working. I did some digging and found better instructions than what i found before for getting MiniDLNA running on FreeNAS. Not sure why they make it so difficult to set it up. You would think that once someone on their forum put together some decent instructions they would make an easy to find knowledge base articl