PayPal IPN are failing

I am using PayPal with Amember. When someone pays via PayPal, amember is not notified that a payment has been completed and their membership is not activated and must be activated manually.

I have received an error from PayPal:

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

http://www.mysite.ca/amember/plugins/payment/paypal_r/ipnr.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.

When I look at PayPal the IPN setting is:

Notification URL http://www.mysite.ca/amember/plugins/payment/paypal_r/ipn.php

notice there is no 'r' in inpr.php.

Either way, both URLs fail.

I get a 500 Internal Server Error that directs me to the server error log but I can't find a server error log.