PayPal Instant Payment Notification warning

I got an email from Paypal today:

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

If you don’t recognise 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.?

When I access the listener link, the following message shows up:

Empty PayPal IPN Request

“Check this option if you want to send the IPN data to PayPal for validation” is unmarked in the plugin settings.