Paypal IPN Failing

I got the following message from Paypal:
"...Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:
http://www.xxxxxx.com/wp-content/plugin ... -ipn.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. "
What may be causing IPN to fail and how would I know?
Should "https" be used instead?
Thanks.
"...Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:
http://www.xxxxxx.com/wp-content/plugin ... -ipn.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. "
What may be causing IPN to fail and how would I know?
Should "https" be used instead?
Thanks.