Why Am I Receiving PayPal Instant Payment Notification Warning

Every time I am sent a payment, not related to Prosites (for example, a friend sends me money let's say), I get this warning from PayPal:


Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:


If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.


This just started happening about a month ago and I've had prosites setup for over 6 months now. The response code in the IPN history logs for each warning is 404.

ANy ideas?