Paypal IPN notifications are failing

I am getting an error from paypal that "IPNs sent to the following URL(s) are failing:" 
https://mydomain.com/checkout_process.php
and the error seems to be in that file

The money is still deposited in my account but a2billing doesn't complete the top-up.
Does anyone have a solution for this error? The last paypal top-up that worked was Mar 18, 2021.