I currently have four video files that would not play because of error: "file not supported"

Dennis advised that "itís because thereís more than 20 tracks (FFmpeg only supports 20 tracks)"
-- this was correct

-- Dennis also advised: "FFmpeg is meant to remove the track limit at some point in the future"

Dennis from Conceiva recommended downloading software MKVtoolnix (free) which allows you to remove unwanted audio and subtitle streams to reduce below 20 limit of tracks.

I have used this software to fix problem MKV files with 100% success.

This software enabled me to delete the unwanted audio and subtitle tracks and watch the movies with Mezzmo
-- I generally delete all subtitles and foreign languages which makes the file slightly smaller in size.

-- the process takes about 6 to 8 minutes depending on size of video

-- in the bottom half of the screen each file has a track description for audio and subtitles (russian, english, english DTS etc) to assist you in selection for deletion of tracks

MKV tools programs:

Guides on how to use them:

My latest problem videos MKVtoolnix tracks listed were:

Sling Blade (1996).mkv
-- one video, one audio, 23 subtitles and one chapter track (total 26 tracks)

The Guardian (2006.mkv
-- one video, one audio, 20 subtitles and one chapter track (total 23 tracks)

Children of Men (2006).mkv
-- one video, five audio, 19 subtitles and one chapter track (total 26 tracks)

Dirty Harry (1971).mkv
-- one video, one audio 18 subtitles and one chapter track (total 21 tracks)