I have the exact same problem as the OP, but the above posts did not solve the problem. In fact, after entering '/' in the Insert page breaks before (XPath expression)
field the conversion failed. With just the default / in the field, the conversion is successful but Calibre is making unwanted page breaks after the image and before the chapter. (and I haven't a clue what XPath expression signifies)
BTW, the book in question was created with Sigil.
Can someone lend a hand?