XHTML elements in Atom feeds need to have the correct XHTML namespace specified. Compare this version:
<content type="xhtml" xmlns='xmlns="http://www.w3.org/2005/Atom'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<p>...</p>
with the namespaces in your current feed:
<content type="xhtml" xmlns='xmlns="http://www.w3.org/2005/Atom'>
<div>
<p>...</p>