wp-custom-fields-search
で、
Warning: Illegal string offset 'preset' in ××/××/××/wp-content/plugins/wp-custom-fields-search/wp-custom-fields-search.php on line 401
のエラーが出る場合、wp-custom-fields-search/wp-custom-fields-search.phpを
function process_shortcode($atts,$content){
$atts["preset"] = ""; //この行追加
return $this->generate_from_tag(array("",$atts['preset']));
}
とすればよい