Just to state - default theme (2012) and no other plugins except Membership.
After successful registration, the form redirected from /register-2/ (the form page) to /register-2/false which returned "404".
After that, I received an e-mail from Authorize.net, saying the payment has been made (Sandbox account) and checked the back-end. The subscription was active.
After I removed the subscription from that test account, and tried again, I get this error:
Fatal error: Cannot use string offset as an array in /home/***/domains/***.net/public_html/***/wp-content/plugins/membership/classes/Membership/Render/Gateway/Authorize/Form.php on line 116
Just above that error, there is a radio button:
<input type="radio" name="profile" value="2">