Membership Premium payment page (Authorize.net) redirects back to register page.

The credit card collection page, when the payment form is submitted, redirects back to the coupon code page (/register/?action=registeruser&subscription=1).

Using Authorize.net. The ID, Key and MD5 are set. SSL is on and working. All Membership Pages are set.

Authorize.net says they are not receiving any payment information. PHP error logs don't contain anything. Please help.