PayPal Instant Payment Notification Warning

redsautorehab.com
zc2.0
php 8.1

I just got this email from Paypal, and I'm not sure how to proceed. All of the orders seem to go through. Cureently using paypalwpp for paypal payments.

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

http://www.redsautorehab.com/ipn_main_handler.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.

I tend to wonder if there's something going on with the host server, because I have been having an ongoing issue of too many database connection errors, as well as shipstation is completely incapable of importing orders to to a 409 conflict error, that nobody has been able to figure out.

I finally got an error to set for the ipn, it looks like it's an issue with definitions:
PHP Fatal error: Uncaught Error: Undefined constant "EMAIL_LOGO_FILENAME" in /home1/redsauto/public_html/includes/functions/functions_email.php:634

The problem I am running into, is that it was working until recently, and the lang.email_extras.php file has not been altered, and shows 'EMAIL_LOGO_FILENAME' => 'header.jpg',

I think the problem is that the IPN processing (which is its own "flow") loads language files "after" the point where any IPN debug/logging emails are generated/sent.
These changes should fix it: https://github.com/zencart/zencart/pull/7446/files ... particularly the changes in paypal_ipn.core.php where the 71 is changed to 79 and the 95 is moved to 75.

Switching to the newer PayPal RESTful module will avoid all of those IPN issues, too.