Using the last revision from the trunk, I'm not able anymore to create mp4 files within chapters.
command line used:
MP4Box -add video.h264 -add chapters.txt:chap -new out.mp4
MP4Box -add video.h264 -chap chapters.txt -new out.mp4
=> crash at [Chapter Import] guessing frame rate detection...
Could you reproduce and check what is going on ?