I'm trying to sell subscriptions (yearly memberships) using the Simple Commerce module. I've set-up everything on EE side and on PayPal's (including the Instant Notification URL). Since I'm still testing everything, I'm not using encryption. Also I'm not using sandbox, but doing tests with 0,01p payments using real paypal accounts - my business and personal accounts, both verified. My business account is set to accept payments automatically.
When the payment is received, a first instant message to notify the subscription is sent successfully. Then PayPal tries to send another ipn, to notify the payment, but this one gets stuck with a "error 500".
It took me a while to get the first IPN, because one of the 3rd party extensions (VZ Bad Behavior, just in case someone else is experiencing the same problem) didn't allow paypal to hit the url. But if the first IPN gets through, why not the 2nd one?
Also, I'm using Expresso Store on the same install, and never had a problem with IPNs.
Any suggestion would be much appreciated.