Hello, we have a SERIOUS problem with user registration!

We get emails from paypal that new user has paid with Transactionscode and price. But this payment is not in transaction in admin-cp and the new user is not registered!!!

Now we also get warnings from paypal that our IPN domain is wrong. Our IPN-domain is:

https://mydomain.com/payment/post.html?gateway=paypal

but google send us another domain?

What is wrong?

Please help, it's urgend!!