Let me add the following to this issue:
1) My book has 53 chapters (each within its own file), some 500 pages.
2) I did a preflight and no errors.
3) I exported the book as a PDF with no issues.
4) When I try exporting the book to ePub, apparently InDesign is looking for something it can't find to complete the process; creating an endless loop—application not responding. So, I then have to force quit. I let the beach ball run for over 10 minutes with no export happening in the progress bar.
5) Is there a known bug in this instance? Or a setting in the export options dialog box that needs correcting?
A) Digital Editions Export: I've also tried exporting with the default TOC style, but no help there, either.
B) Book palette: After I set up for ePub export (but cancelled), the palette indicates that a number of files are open—they are not!