I assume this is your cover .xhtml page (just from glancing at your content.OPF file) -- see below.
If not, it might be worth adding an actual .xhtml page for it.
Code:
<manifest>
<item id="content_0_item_0" href="64e747ce5301520dc5af4bc7.xhtml" media-type="application/xhtml+xml" />
</manifest>
Code:
<spine toc="ncx">
<itemref idref="content_0_item_0" />
</spine>