Even though payments are coming through, and members are being registered, and everything onsite looks ok, I receive the following email from PayPal every day:
PayPal Instant Payment Notification Warning
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.
Thank you for your prompt attention to this issue.
Now, I have read that there are other IPN threads on the s2member forums here, and am no clearer as to why this might be happening. If I check my logs I see:
gateway-core-ipn.log:  => Storing IPN signup vars now. These are associated with a User’s account record; for future reference.
Which seems to imply everything has been successfully processed. What is PayPal’s (or my) problem?
ps. I’m using s2member Version 200301 + s2Member Pro v200301