That's PayPal picked up X-Payments callback script as your main X-Cart callback script and tries to use it for other (non X-Payments) PayPal transactions as well.
The solution is to specify your X-Cart callback script as the right one in your PayPal account settings. See
https://help.x-cart.com/X-Cart:Enabling_Instant_Payment_Notification_%28IPN% 29_in_PayPal_account