Categories no longer pull up posts

Beginning with WP 3.1, the categories on all my subsites will not pull up the posts linked to the categories. Categories will pull up pages but not posts

I am using the Twenty-Ten theme.

To see the problem, go to

and click on some categories on the sidebar. It takes you to a search window which works fine. Notice how the footers below the posts indicate the categories.

Thanks for any ideas,