I've suddenly begun getting emails from Paypal saying that our Express Payment option will be revoked if we don't fix a problem. I see that there are several threads about this in the past, but the most recent is several years old and not something that would apply to us. Nothing on the website has changed recently, so I don't understand why we're suddenly getting this notice.

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


https%3A%2F%2Fwww.newnaturalsonline.com%2Fipn_main_handler.php


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.
I have looked at the url referenced in the email, and ipn_main_handler.php does exist there. I've also looked at the logs and don't see any warnings or errors relating to the IPN handler. Paypal doesn't say how it's failing, so I don't know what else to look for. Can anyone help?