Hi Nicolas,
I've managed to drag some more information out of the client:
She was using Safari(Mac OSX). I just tested in Safari on a Mac as well here's what happens:
- when I click 'next' on the first checkout step, the step 2 opens in a new tab
- when I click 'next' on step 2 in the second tab, a third tab opens. In this third tab, it shows me the page of step 2, but the link in the address bar shows step 3
My checkout workflow is: 1. Login, Address -> 2. Coupon, Cart, -> 3. Payment, Shipping, Status, Cart, Terms and Conditions.
This problem only seems to happen in Safari; it's fine in IE, Chrome & Firefox.
Does this information help at all?