IPN problem message from Paypal

Been using S2 with Pay Pal for 2 years - we have not changed anything:

Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:
https://www.wfahweb.com/?s2member_paypal_notify=1

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.