Issue #3313467 by Defcon0: PHP 8.1:

responsive_image_formatter_link_to_image_style.field.inc

@@ -42,7 +42,7 @@ function template_preprocess_responsive_image_formatter_link_to_image_style_form
   $attributes = [];
 
   // Do not output an empty 'title' attribute.
-  if (mb_strlen($item->title) != 0) {
+  if (mb_strlen((string) $item->title) != 0) {
     $attributes['title'] = $item->title;
   }