Don't use WP_PLUGIN_URL.

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().

To summarize:
In wp-insert.php
line:
define('WP_INSERT_URL',WP_PLUGIN_URL.'/wp-insert');
replace with:
define('WP_INSERT_URL',plugins_url().'/wp-insert');

http://wordpress.org/extend/plugins/wp-insert/

P.S: See also: WP_PLUGIN_URL doesn’t take SSL into account