I tried to merge a subtitle SRT file with a video MP4 file using MKVMerge as explained here. It failed with this error:
$ mkvmerge -o out.mp4 foo.mp4 foo.srt mkvmerge v5.6.0 ('Kenya Kane') built on Jun 7 2012 15:57:56 'foo.mp4': Using the demultiplexer for the format 'QuickTime/MP4'. Error: The demultiplexer for the file 'foo.mp4' failed to initialize: The file could not be opened for reading, or there was not enough data to parse its headers.
Googling for this error message did not provide any solution.
I updated MKVMerge to the newer version 6.7.0. The same command on the same files worked successfully! 😊