Suddenly unable to access plugin-specific settings

Any plugin. All of them.

Debugging info:

You do not have sufficient permissions to access this page. 

Array
(
    [Pages No Access] => Array
        (
            [edit-tags.php?taxonomy=link_category] => 1
        )

    [User Info] => WP_User Object
        (
            [data] => stdClass Object
                (
                    [ID] => 1
                    [user_login] => xxx
                    [user_pass] => ZZZZZZ
                    [user_nicename] => xxx
                    [user_email] => EMAIL
                    [user_url] =>
                    [user_registered] => 2011-12-28 22:36:10
                    [user_activation_key] =>
                    [user_status] => 0
                    [display_name] => xxx
                )

            [ID] => 1
            [caps] => Array
                (
                    [administrator] => 1
                )

            [cap_key] => myprefix_capabilities
            [roles] => Array
                (
                    [0] => administrator
                )

            [allcaps] => Array
                (
                    [switch_themes] => 1
                    [edit_themes] => 1
                    [activate_plugins] => 1
                    [edit_plugins] => 1
                    [edit_users] => 1
                    [edit_files] => 1
                    [manage_options] => 1
                    [moderate_comments] => 1
                    [manage_categories] => 1
                    [manage_links] => 1
                    [upload_files] => 1
                    [import] => 1
                    [unfiltered_html] => 1
                    [edit_posts] => 1
                    [edit_others_posts] => 1
                    [edit_published_posts] => 1
                    [publish_posts] => 1
                    [edit_pages] => 1
                    [read] => 1
                    [level_10] => 1
                    [level_9] => 1
                    [level_8] => 1
                    [level_7] => 1
                    [level_6] => 1
                    [level_5] => 1
                    [level_4] => 1
                    [level_3] => 1
                    [level_2] => 1
                    [level_1] => 1
                    [level_0] => 1
                    [edit_others_pages] => 1
                    [edit_published_pages] => 1
                    [publish_pages] => 1
                    [delete_pages] => 1
                    [delete_others_pages] => 1
                    [delete_published_pages] => 1
                    [delete_posts] => 1
                    [delete_others_posts] => 1
                    [delete_published_posts] => 1
                    [delete_private_posts] => 1
                    [edit_private_posts] => 1
                    [read_private_posts] => 1
                    [delete_private_pages] => 1
                    [edit_private_pages] => 1
                    [read_private_pages] => 1
                    [delete_users] => 1
                    [create_users] => 1
                    [unfiltered_upload] => 1
                    [edit_dashboard] => 1
                    [update_plugins] => 1
                    [delete_plugins] => 1
                    [install_plugins] => 1
                    [update_themes] => 1
                    [install_themes] => 1
                    [update_core] => 1
                    [list_users] => 1
                    [remove_users] => 1
                    [promote_users] => 1
                    [edit_theme_options] => 1
                    [delete_themes] => 1
                    [export] => 1
                    [NextGEN Manage tags] => 1
                    [NextGEN Manage others gallery] => 1
                    [access_gtmetrix] => 1
                    [wpseo_bulk_edit] => 1
                    [administrator] => 1
                )

            [filter] =>
        )

    [Roles] => Array
        (
            [administrator] => Administrator
            [editor] => Editor
            [author] => Author
            [contributor] => Contributor
            [subscriber] => Subscriber
        )

)