Here is something from the forums:
I don't know your exact problem, but here is some advice about using MKV (and other ripping SW):
Clean/fix your discs-
I wipe my discs with a dry cloth, if there is visual dirt on them that won't wipe with a dry cloth, I wash my discs in a sink with dish soap and even a sponge if needed.
I get a lot of movies from stuff like craigslist, so the condition of them are not great, so sometimes I need to not only clean them, but also repair the scratches. I use car wax for that.
I also ran into issues where a literal perfect looking disc wouldn't play. The only think I can think of is that maybe the disc was exposed to heat or light, or both. I still haven't found a solution for this one.
The drive-
With some discs, sometimes one drive might work better than another. Sometimes just taking the disc out of a drive and sticking it back in helps with errors.
If you are using a portable drive, use a USB cable with a separate USB power plug.
Clean the drive as well, especially if it has never been done before. Drive cleaner discs are really cheap.
The SW-
If one fails, use another, especially for DVDs. If you cleaned the discs, clean the drives, used different drives, and still not working. Try using different SW. I have done this a few times, MTR is my second go-to.
AFAIK, MTR isn't updated anymore, but it still works great (not sure about newer OS versions). I have been using MTR for what seems like 20 years.