Since the latest update the file symbols.php has apparently been copied from the plugin Collapsing Categories. On line 10 and 13 the paths to expand.gif and collapse.gif contain a /wp-content/plugins/collapsing-categories/ part. This must be /wp-content/plugins/collapsing-archives/ otherwise the gifs cannot be found and the alt text is displayed.
https://wordpress.org/plugins/collapsing-archives/