Alrighty, let's try this. Can you disable all of your plugins (temporarily) and then visit one of the parent category pages to see if it's working now?
If so, one of your plugins is causing this. You can find out which one by enabling them one at a time and checking the category page again after each new plugin is enabled. Eventually, you'll find the plugin that's responsible for the break and we can go from there.
If disabling the plugins doesn't fix it, then I'm afraid I don't really know anything else to do. :/