[Plugin: BackUpWordPress] wp-cron.php is returning a 403

I am receiving the following error: wp-cron.php is returning a 403 and I have tried adding define( ‘ALTERNATE_WP_CRON’, true ); to my wp-config.php
file with no luck. I can do a manual backup but can not do scheduled backups. Any help would be appreciated.

http://wordpress.org/extend/plugins/backupwordpress/