Just updated to the latest version of WordPress & SEO plugin.
It caused some category pages to not be navigable.
Deactivated all plugins and activated 1 by 1. SEO plugin seems to be the culprit and throws the following when trying to reactivate:
Plugin could not be activated because it triggered a fatal error.
Fatal error: Cannot use object of type WP_Error as array in /hermes/bosweb26a/b1655/ipg.samuipediacom/wp-content/plugins/wordpress-seo/inc/wpseo-non-ajax-functions.php on line 136