It won't open .mkv files that are more than one day old. These .mkv files used to work, but now they don't. However, I can still open .mkv files that I had from yesterday. I tried deleting the file "org.niltsh.MPlayerX.plist" but I can't drag it anywhere and there's no option to move it to the trash when I right-click it. And yes, I made sure that Mplayerx was not open. Thanks.