What wrong with paypal and whmcs ?

Just received

PayPal Instant Payment Notification Warning‏

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

We have used same IPN and haven't change any setting for 5 years + it is working fine before, it start receiving that kind of email 1 week ago.

Do you have guys know that, how to get this solve ?:?: