Hi,
Thank you for your reply. I tried setting the IPN URL to "YES" but it did not help. Invoice is still not marked as paid and I'm not redirected to the payment complete page but back to the invoice I just paid.
The transacation on PayPal's end went through ok. PayPal IPN history shows that the message was sent to the right notification URL and the HTTP response code is 200.
I am using the sandbox mode and have double checked the PayPal usernames.
How can I debug this? Anything else I can try?