Ubercart

I'm having the same issue. I went digging last night and found that the $_SESSION['cart_order'] value is getting smoked somewhere between function uc_cart_checkout_review and function uc_cart_checkout_review_form_submit, so there is something that is hooking into the form and unsetting $_SESSION['cart_order'].

What's interesting is that I put in a little workaround that seems to work pretty well barring any unforeseen negative effects - I pass the cart_order value into another session variable within uc_cart_checkout_review, and pass it back into cart_order in uc_cart_checkout_review_form_submit under the case of Submit order submission.

I found the error occurring on every payment method I tried (COD/Check/Auth.net/CC/Other). I do have a small block that I'm using to display a certain product if it doesn't occur in the cart, but a particular other product does (a bit like 'this goes really well with that'), which calls uc_cart_get_contents, but that's about it. It's confined to the cart view page anyway. I have:

Donation / donation product
Recurring
Payment pack (for testing)
CC
Authorise.net (thanks for the ARB, Ryan)
Secure pages (error occurs with this disabled too, and it shouldn't make a difference because the form submission to checkout is all secure, so there is no session problem)

Some additional notes:
1) Only $_SESSION['cart_order'] gets cleared between cart/checkout/review and cart/checkout callback.
2) A new order is generated when returned to cart (although the old one is still in the order table), probably because the session variable gets wiped.
3) This is a new site, still under development, and the first couple of orders went through, I just don't know what was added that could be causing the problem. I thought it might have been Auth_CIM, but I fully uninstalled and deleted the module (although it doesn't drop the CIM tables - naughty naughty), so there are no function that can be invoked there.

- $Dave = array('state' => 'officially stumped');