Since upgrading to 3.1 I have noticed a problem on one of the sites in my WordPress network.
I have a category which contains posts and a subcategory. When I visit the category's archive page I get the loop error "Sorry, but you are looking for something that isn't here." If I remove all the posts from the subcategory, the category displays properly.
This particular site has no plug-ins enabled (though other sites on the network do).
It's weird, right? Any ideas?