DVD created with ffmpeg and dvdauthor doesn't start at the beginning
I've resorted to using mkisofs to create the ISO image, and this seems to fix the problem. Creating the ISO in another way (e.g. on a Mac with hdiutil, or directly burning it to disc with Finder) creates malfunctioning DVDs. Apparently there are some restrictions on the ISO format for DVDs. If these are not respected, the DVDs have incorrect chapters and might even start in the middle of the video. These restrictions are best explained in mkisofs' man page:


  -dvd-video
  Generate  DVD-Video compliant UDF file system. This is done by sorting the order of the content of the appropriate files and by
  adding padding between the files if needed.  Note that the sorting
  only works if the  DVD-Video filenames include upper case characters
  only.