This error message is produced by the Joomla simplepie library which makes it possible to read from an RSS feed. The "Latest News" panel in the Projectfork sidebar is reading from such a feed. The feed URL has changed in Projectfork 2.1.3. If you are using an older version, Simplepie will throw this error.

To solve it, navigate to: Projectfork -> Config -> Panels and edit the "Controlpanel news" panel.
Then change the Feed URL to "http://feeds.feedburner.com/projectfork/blog".