A broken link scan of our website revealed that the link in the feeds to http://wellformedweb.org/CommentAPI/ (for the wfw namespace) is invalid. Going to that website reveals that the site has been taken down by the hosting company.
I found 2 references to that namespace in /wp-includes/feed-rss2.php and /wp-admin/includes/export.php and removed them, and the instances to the wfw namespace within the feed content. This fixed my issue.
This begs the larger question: should this be removed/replaced in the core if the site that references the namespace is no longer available?