I've been using the OnestepCheckout extension for Magento with success for a long while.
But now I want to set up multiple store views. I set this up correctly, but when I add a store code to the urls, the checkout page empties my cart.
domain.com -> domain.com/onestepcheckout works fine.
But:
domain.com/en -> domain.com/en/onestepcheckout clears the cart
How is this possible? Has it something to do with the setup for cookie domains?
Most probably you are loosing your session during redirection time so monitor your front-end session id and see if this changes. If it changes then you are loosing your cart.