I've got another situation where my source feed validates and my FeedDigest feed doesn't
The issue is in the first article. The source feed has a link to a Joomla component and the link isn't valid in Feed Validator. In my source feed the article is enclosed in CDATA, the FeedDigest removes the CDATA.
I could fix this by creating a redirect in my htaccess file and not using this Joomla link, but I tend to use these kind of links a lot, so I'd be interested in discovering if this is a bug.