The site I'm working on right now is an existing installation that I inherited and have recently upgraded. Unfortunately, it doesn't seem that I can view categories on the front-end properly: when trying to go to a category, it just shows the home page.
Some quick debugging code showed that it is in fact loading the index template, and thinks that it has no query data for WP_Query. is_home() returns true. The site is here (a category link):
What's also strange is that the /blog/ part is in the post permalink structure, but is not the base for category links. It shouldn't be there, yet that's the link that WordPress generates. Removing that fragment renders a 404 page.
It seems to be a permalink structure problem, whatever it is. Setting the URL structure allows the category to show up just fine. What might be causing this?