PayPal Instant Payment Notification Warning email

Hello, I found a few previous posts about this, but none seemed to have addressed the problem.

I received the following email 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://mydomain.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.

Thank you for your prompt attention to this issue.

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I have been using Pro Sites for a few months and have been getting payments fine. How should I address this issue to be sure to continue receiving reoccurring payments?

Thanks, Chris