Some "sources" in Cydia are empty - how to get the content?
Aha, it turns out that the sources have moved. They're no longer located as specified in Cydia, so of course Cydia can't find any content.

The solution was to remove the faulty sources and add them again by following these instructions: http://www.appleiphoneschool.com/how-to-add-a-source-to-cydia/