I am having difficulty opening an MPEG-4 video that I downloaded from Vimeo even though I understand that MPEG-4 files should open in QuickTime. I keep getting the message "The file may be damaged or may not be a movie file that is compatible with QuickTime Player" even though it plays perfectly on Vimeo. I would insert this MPEG-4 file but the icon for "Insert Video" is grayed out above.
Can't do that (upload) anyway. Videos can only be added (where available, and this forum ISN'T one where it can be done) by using an "embed code".
The message you're seeing indicates that the file was "corrupted" during the download and either the header ot the leadout of the MP4 is no longer intact. Some video downloaders do this.
Quick Time is "quirky" when to comes to playback. MOV files are what work best with it, even though MP4 should play just as well. I have several MP4s that won't open in Quick Time but VLC Media Player has no trouble at all with them. Give it a try.