thanks - unfortunately it doesn't help. Interesting enough is though that I can see the 302 HTTP request of "checkout.php" - and it does redirect to order_info.php - but doesn't seem to pass on the cart.
Could it be related to some php setting?
By the way: I was using a .htaccess (as I don't want anyone crawling around on a test site), changed it, removed it, replaced it etc... unfortunately without a positive outcome.