Yes. it is because the post has <![CDATA[ … ]]> in it.
Use CDATA in RSS Feed to add HTML and links | Amit Tech Lab - PHP, AJAX, CakePHP,WordPress, Symfony, Drupal, codeIgnitier
This results in something like
<description><![CDATA[ … <![CDATA[ … ]]> … ]]></description>
The validator / parser doesn’t do nested CDATA well.