Order Received is Showing Page Not Found

Hi!
I have had this issue for weeks. Cant find a solution. Anyone?

Thanks

Here is the mail from Paypal;

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

https://jonsavagegallery.com/order-received

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.