This is the same issue as I reported.
www.rockettheme.com/forum/gantry-for-joo...-load-renderer-class
I can confirm that following the steps
1. get any invalid URL of your website (http://yoursite(dot)site/some-ugly-page)
2. add to it to ?format=raw or ?format=json or ?format=xml
Will cause this error.
This does not occur with default Joomla templates.
I have not found a resolution to the issue.