Today a feed for one of our blogs mysteriously stopped functioning. After agonizing hours of checking server environment, FeedForAll feed reader script and the integrity of the WordPress database, we finally stumbled on the Feed Validation too:
It told us that:
1. The feed was not valid
2. The "»" character that someone had added into the title of the blog was the culprit.
I include this in the hope it may aid others.