simpleCart "Not a valid PayPal IPN request"

Hello, All -

As my client's web shop is starting to pick up business so he is starting to experience repeated instances of "Not a valid PayPal IPN request" error reports. The transaction is completed normally on PayPal but for some reason unknown the IPN script then seems to hit a brick wall. As a result the sale is not logged as "Paid" in simpleCart and none of the notification emails fire.

My client is getting annoyed. What could be causing the problem?

:0/

Perch