You probably used an unencoded reference movie into DVDSP rather than a "self-contained" movie.
A reference movie is much smaller since it simply points to all the existing parts of your movie: the original capture plus any rendered effects or transitions. But it therefore requires that all those parts are still around. Render files in particular seem to get deleted or changed such that a reference movie no longer recognizes that they're there. You then get the message about "the movie file 'FIN-000000' cannot be found." The strange file name is a render file that's now missing. You will get the same message if you try to open the reference movie in QuickTime Player.
The fix to let you open your DVDSP project is to delete or move the reference movie you placed into DVDSP. DVDSP will then say "I can't find the movie" and let you either replace it or ignore it, then proceed to open the disc project.
The long-term solution is to either use self-contained movies (which don't result in this problem) or to encode to MPEG2 prior to placing into DVDSP.
