Hey @Repofmancunia1 - as far as I'm aware, the best solution here would be to convert your MKV files to MP4 or another format. You should be able to find a free converter online if you don't have one already.
That's certainly strange @Repofmancunia1
If you haven't done so already, are you able to perform a soft and/or hard reset of your set top box to see if that helps? You can find a step-by-step guide on how to do this here → https://goo.gl/LxQy4A.
Let us know how you go.
Thanks GuyCS. I did the soft reset. The Fetch box also loaded Software Update (2.20.1 Freycinet). Not sure if that was part of the reset or just doing its own thing. The choppy playback appears to be cured and I can access the movies on the Plex Media Server - however usually have to select the item at least twice before the folder or selecetion actually appears. The music selection is a bit hit and miss (it plays some files, others just skips a whole folder). The photos folder --- takes a while loading then shows two dots for a few seconds, then shows the complete photo for maybe one second or less then moves on to the next photo repeating the cycle.
Updates are pushed when they're available @Repofmancunia1, may of just been a coincidence. It's great that it seems to have resoled the playback problem! The issue with browsing and selecting folders as well as viewing and playback some photos and audio, may just relate to either your local network or the specific compression of the files themselves.
I'd maybe try testing a few files you're having trouble with by putting them on a USB drive and accessing them that way. If they work OK, could just relate to your Plex server set up or your network. If you need further assistance, might be worthwhile getting in touch with Fetch, as advanced features like this are outside of our support boundaries. For a full list of supported file types, check out the Fetch Media Hub quick start guide