MKVToolnix muxing problem when adding or removing audio and subtitles

[firstliight]

First I will try to explain what I've been trying to do and then the problem I am experiencing. I like to rip all my movies to .mp4 files because I can then add the cover art. The tool I use to convert to .mp4 is AVS Video Converter, it converts just about any video format flawlessly, I love it. Sometimes I acquire video files that are .MKV files that sometimes need to have English subtitles, or a additional English audio added and AVS Video Converter can not add to a MKV file so in that case I was lead to MKVToolnix which is suppose to allow one to add or remove audio and subtitles and it does that, but! after I have done the addition of subs or audio to the file and then rename it, and then click start muxing everything seems to complete just fine, and even the program says everything completed with no errors.

The problem is then noticed when I try to take the new file and add it to AVS Video Converter to convert it to .mp4, the new remuxed MKV Video file will not even load into AVS Video Converter at all and displays a error message that reads "File format error, but the original MKV file will load without any hesitation, so I am assuming I am doing something wrong when I added in new audio or subs.

So I have come here to find out what I am doing wrong and what I can do to correct this problem.

Thank You

[Baldrick]

Try mkvtoolnix 3.4.0 or older . Or try disable header compression in mkvtoolnix 4.0 or newer.