Please DON'T use "WP_PLUGIN_URL." in wp-insert.php because it is not SSL aware. When in the back-end admin panel via HTTPS (all the time if FORCE_SSL_ADMIN is set!), instances cannot be set via lightbox confirm button, because lightbox menu is called via http.
Instead use: plugins_url().
P.S: See also: WP_PLUGIN_URL doesn't take SSL into account