Ok, tried that, now it seems the chapters are defined by:
<div class="mbp_pagebreak"/><blockquote class="calibre_24"><blockquote class="calibre_25"><font size="5">CHAPTER 1</font>
</blockquote></blockquote>
But I've tried setting Structure Detection to "div", "class" and "mbp_pagebreak" as well as "blockquote", "class" and "calibre_24", still no luck.
I also tried just leaving tag as * and attribute blank, and value as "calibre_24" (also tried "calibre_25", still nothing