A couple more bits of data:
1) This is the error message I'm getting from ID. I always get the "Custom Layout settings..." message, so it can't be that. The "External Text Anchor..." message might be the problem, but I have double-checked every link in the document, and they're all good.
2) After I click "OK" on the "Book Conversion Failed" dialog, Kindle Previewer opens this Excel file, which I've never seen it do before, and which I cannot comprehend, especially as it contains the contradictory message "Book conversion successful":
