Quote Originally Posted by DrByte View Post
Broken /includes/classes/shopping_cart.php file. Most common problem is using an addon that contains an old version of the file, instead of the new version.
My version of /includes/classes/shopping_cart.php is correct. Still looking (frustratedly) for other addons that might need updating.

Could it be that I'm using the (deprecated) cc module that is causing this error?

(This is a problem, since when a new customer fills the shopping basket, then signs on to get to the checkout, they are greeted with the rude "Whoops! Sorry, but you are not allowed to perform the action requested.")