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:

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

http://onlinechimpwebsitesolution.com/wp-admin/admin-ajax.php?action=psts_pypl_ipn

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.

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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?