Using PHP 5.4.12 and zen v151
During checkout: Step 1 of 3 - Delivery Information
Click: Change the Address
Change the Shipping Address: add a new address or choose another.
Click: Continue, goes back to Step 1 of 3 - Delivery Information

At this point I get the error:
PHP Warning: Illegal string offset 'cost' in /home/XXXXXXX/public_html/includes/classes/order.php on line 355

Quick temporary fix. Find line 355
PHP Code:
  'shipping_cost' => $_SESSION['shipping']['cost'], 
Change to
PHP Code:
 'shipping_cost' => (isset($_SESSION['shipping']['cost']) ? $_SESSION['shipping']['cost'] : 0),