If you're really stuck, you can email hello@leanpub.com and we'll try to help.
But before you do that, can you try this?
Download an archive of the most recent success and most recent failure.
Try replacing the manuscript with the most recent success and preview again, as a sanity check.
Assuming it works, replace the manuscript with the most recent failure and preview again, as a sanity check to ensure it still fails.
Compare the two manuscript directories (using diff, if you're a programmer) and see if anything jumps out.