You need to allow access on /wp-admin/admin-post.php

WordPress provide two access on wp-admin for not logged in users : admin-ajax.php and admin-post.php. Both can fire a hook for an action _nopriv.
Actually, the plugin allow to access admin-ajax.php, but block admin-post.php in your code :

//case where someone attempting to reach wp-admin 
        if (is_admin() && !is_user_logged_in() && !defined('DOING_AJAX')){
            //Fix to prevent fatal error caused by some themes and Yoast SEO
            wp_die( __( 'Not available.', 'all-in-one-wp-security-and-firewall' ), 403 );
        }

Please update this code ??