I noticed that when I upgraded WP to 3.5, then feed (containing images from YAPB) broke. The embedded image tag was not closed.
I've tried ticking the XHTML style box in the feed settings, but thats made no difference.
I can almost pin-point the issue occurring when upgrading to 3.5 when it was released earlier this month as the feed items before that date were rendering fine. Would this be an issue with YAPB? Or is WP modifying the output of the RSS?