My payment gateway returns a 500 error, can you debug it?

Yes. A 500 on a Stripe, PayPal, Mollie, or custom gateway callback almost always traces to the webhook/IPN handler, an expired API key, a changed API version, a malformed response, or a PHP fatal inside the handler. We read the gateway logs and the server error log, fix the handler, and verify with a real test transaction that the order is created and marked paid correctly.