Exactly teh same problem at my site.
I think it depends on phpThumb.php:
$content_dir = str_replace('/', '\/', str_replace( ABSPATH, '', WP_CONTENT_DIR ));
preg_match('/\/' .$content_dir .'\/([0-9\_a-z\/\-\.]+\.(jpg|jpeg|png|gif))/i',$_GET['src'],$match);
$image_name_clean = $match[1];
$extension = $match[2];
This is not working for a url with "files" instead of "wp-content".
Please fix this.
Regards Benedikt