"Unauthorized Action" when trying to browse Category

I’m having an issue with a WordPress site I’m running. I never noticed this before but a user pointed it out today, so I’m not sure how long is has occurred. I know the categories used to work properly, so I don’t know if it has to do with the new 3.3.1 upgrade. The website is http://gigafytes.com, and across the top there is a navigation bar with links to specific post categories. When trying to navigate to any category, I get an error page saying “Unauthorized Action”. It only happens for users who are not logged in (aka, the most important visitors to the site!). When contributors log in, the links work fine. So basically, logged in users can view posts by categories, and anonymous users get the error message. Any ideas?