I am adding some additional information from the debug files, and I hope someone can give me some pointers as to how to correct this problem.
This is the parsed/index.html:
Spoiler:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title/>
<meta content="http://www.w3.org/1999/xhtml; charset=utf-8" http-equiv="Content-Type"/></head>
<body>
<p>第1章</p>
<p class="softbreak" style="margin-top:.5em; page-break-before:avoid; text-align:center">*</p>
<p>
And this is the structure/index.html:
Spoiler:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml"
id="calibre_toc_1" style="page-break-before:always">
<head>
<title/>
<meta content="http://www.w3.org/1999/xhtml; charset=utf-8" http-equiv="Content-Type"/></head>
<body
id="calibre_toc_2" style="page-break-before:always">
<p
id="calibre_toc_3" style="page-break-before:always">第1章</p>
<p class="softbreak" style="margin-top:.5em; page-break-before:avoid; text-align:center">*</p>
<p>
and this is the structure/toc.ncx: