take a look at the incredibly useful epubcheck validator from threepress.org :
http://threepress.org/tools/ it will show errors, and there are some explanations on the side of the page. i just ran your epub through it and there's an un-closed H1 tag in your content004.xhtml file (line 13). that could be causing the problem.
sigil is at the very very beginning of its development, but for a completely free, open source, community-feedback-based app developped by "an overstressed and overworked grad student who doesn't get nowhere near the amount of sleep he should" for sheer love of the art (and love of epub, presumably, and maybe some other crazy ideas), i think it's already a pretty amazingly useful app.
i suspect if you take a bit of time to see what it can do you'll start to like it a lot too, despite its current shortcomings.
it's a completely different beast from calibre, by the way ; calibre is a converter, whereas sigil is an editor.