Category listings broken

Until recently, my category URL (/blogs/category/[category-slug]/) was working fine, using a category.php template. Now, it’s just displaying my index.php template. When I dump the value of $wp_query, I get the following strange results:

object(WP_Query)#206 (42) {
[“query_vars”]=>
array(57) {
[“category_name”]=>
string(7) “example”
…
[“tax_query”]=>
object(WP_Tax_Query)#183 (2) {
[“queries”]=>
array(0) {
}
[“relation”]=>
string(3) “AND”
}
…
[“is_category”]=>
bool(false)

So, as far as I can tell, something’s going wrong between the matching of the URL ‘parameter’ to an actual category. Does anyone know what might be going wrong, or how I can go about debugging this? I’m not sure if the recent upgrade to 3.1 has caused this issue.