title_body: What does FlightCrew's "Unable to find zip structure: End-of-central-directory" mean?
upvoted_answer: That would mean your EPUB does not have a proper ZIP file structure. 
  
  You can try listing the content of your file with a normal zip unpacker, or test it with a real epubreader. If those do not complain, it could mean that FlightCrew has a bug.