Don't forget to flush the relative_url_cache when updating the slug.

modules/gallery/helpers/gallery_installer.php

@@ -479,6 +479,7 @@ static function upgrade($version) {
         db::build()
           ->update("items")
           ->set("slug", $new_slug)
+          ->set("relative_url_cache", null)
           ->where("id", "=", $row->id)
           ->execute();
       }