We have a weird problem. Everytime a customer goes to the checkout page, enters his address and click continue, the customer is redirected to the "Cart Empty" page. This happens for logged in user too - they get logged out. We have tried increasing cookie lifetime to 1 day instead of 1 hour, even a year, but with no success. Right now we have another onepage checkout module installed, and that works fine, but we would really like to get the original design working.