How to Resolve PayPal Instant Payment Notification Warning

I’ve successfully used this plugin for over a year but have recently received email messages from PayPal warning of failed Instant Payment Notifications (IPNs). I contacted my host, HostGator, about the issue but they said I needed to ask the plugin author.

Below I’ve copied the PayPal warning message. Any advice for a solution to this problem?
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Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:

https://www.mysite.com/wp-content/plugins/wp-ultra-simple-paypal-shopping-cart/paypal.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.
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https://www.ads-software.com/plugins/wp-ultra-simple-paypal-shopping-cart/