XHTML output fails

I am looking for errors in my CSS for print. I found a tip to look at the XHTML (PDF Export & Options – Pressbooks User Guide - at the end of the article).

When I tried to view the XHTML, I got this page: https://kalico.pressbooks.com/format/xhtml?debug=prince

This basically represents the first few pages, then starts repeating one partial page over and over again for the rest of the output. I don’t see any XHTML, it just looks like a PDF document.

Any idea what I did wrong here?