Actually the conversion was not the problem, but the inability to edit the resulting azw3 file.. Namely, this section of the metadata.opf couldn't be edited by Calibre:
Code:
<guide>
<reference type="toc" href="text/part0010.html#id_TOC_250019" title="Table of Contents"/>
<reference href="images/00010.jpeg" type="cover"/>
<reference href="images/00010.jpeg" type="other.ms-coverimage-standard"/>
<reference href="images/00010.jpeg" type="other.ms-coverimage"/>
</guide>
... all changes were omitted, and only after several attempts did it edit the azw3.
I edit it so that the TOC can be reached from Kindle's menu (Go to-TOC)... The problem was that the azw3 was
denied multiple editing attempts by Calibre...
Why?