Because proplayer requires WordPress core files directly using
require_once, it is incompatible with my plugin, WP-United.
WordPress isn't always bootstrapped through wp-config, so assuming that
require_once will only bootstrap WordPress is WordPress is not already loaded is incorrect.
To fix the problem, please wrap your call to
require_once(... wp-config.php ...) in a check to see if ABSPATH is already defined, or if this is an AJAX call, use WordPress' AJAX API.
More info here: