Paypal IPN failure message (Membership+ Event+ Plugins installed)

Email from Paypal:
>>>>>>>>Start clipped 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.ctaep.org/paypalipn.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.

Thank you for your prompt attention to this issue.
>>>>>>>END Messgae from Paypal>>>>>>>>>>

This was working fine for a while

Notification URL set on Paypal side: http://www.ctaep.org

We are using payment through Registration+ and Event+, It seems like error is on Registration+ transactions.

IP History failures :-
On Failure Notification URL: http://www.ctaep.org/paypalipn.php
HTTP Response Code: 404