It means that the IPN handler (a script that listens for payment notifications from PayPal, generally used to update shopping carts or other backend admin work) is no longer available on your website.
You should consult with your website admin to rectify the issue.
It's not always the ipn script that makes this happen.
If you use IPN but also give out the same paypal address for manual payments the IPN process will try to send back order details to your site and you IPN script will fail simply because the is no order to process.
So you need two paypal email addresses one for the site orders and one for offline payments if seems.
