I am using the authorize.net gateway in sandbox mode. I have a developer account and valid keys for the sandbox mode.
When I attempt to complete step 2 with one of the test card numbers authorize provides (370000000000002 for AMEX) the form submits, no errors are seen, and a hash appears at the end of the url.
If I logout and log back in, every page redirects to the subscribe page which displays only
"Completed: Thank you for joining It looks like you are already a member of our site. Thank you very much for your support.If you are at this page because you would like to create another account, then please log out first."
In the admin area, I can see that the new user is successfully added along with a subscription and paid level.