This morning Belgian user Benny Fajwlewicz reported a bug that caused Spanning Feed Builder to fail to load for organizations with a large number of custom objects. A fix for the problem was quickly created, tested, and applied to the live server. If you've encountered this problem, just log out of salesforce.com and log back in (to flush the Feed Builder cache), and the problem should be gone.
With the help of users like Benny, we're continually making Spanning Salesforce and Spanning Feed Builder even better. If you find a bug, please let us know by emailing firstname.lastname@example.org. Thanks!