The file /usr/share/perl5/Slim/Formats/Movie.pm contains an error that affects mp4 playback.

The line number varies from version to version but in 7.6.1 - r33110 the line number is 172

return ${ delete ${*$fh}{_mp4_seek_header} };

should be...

return delete ${*$fh}{_mp4_seek_header};