If you're really stuck, you can email firstname.lastname@example.org and we'll try to help.
But before you do that, can you try this?
- Go to https://leanpub.com/YOURBOOK/versions
- 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.