Redirected to WHMcs home after logging in

We've been working on making the order wizard work outside our WHMcs installation, at another WP with MU provisioning installed.

At http://goo.gl/A9MDMg, we use a encoded URL to process a 1 month trial. The order wizard looks fine but at the last step, if you login with an existing account instead of creating a new one, we are just redirected to WHMcs home, without any order placed.

Do you know what could be causing this?