Recover broken kazam video file (.mux)
the answer to this question was posted by bukkojot as a comment to this question on 20171012

github.com/bookkojot/mp4fixer


This tool fixes exactly this problem. I tried it and it worked for me.
There are a few simple steps involved, instructions are enclosed. You need a short properly saved video on the same machine with the same kazam to use as input so it knows what the headers should look like. And you get the mp4 from the temporary movie.mux.