When I want to validate my cart, and when I want to use a saved address for my billing address I have the following address :
An illegal choice has been detected. Please contact the site administrator.
Moreover the copy of the selected address doesn't work.
It only occurs when the order contains a product that works with uc node checkout module. I also tested with a non shippable product not working with UC node checkout and the error doesn't occur.
It's not a blocking error so that if I manually enter my address and validate my cart, it works.