We're sorry. An error occured while processing your order...

I kept hitting this too, on a site with COD and PayPal enabled. Using Free Order instead of COD made no difference, so I sadly had to kill a kitten to get free orders through the checkout.

The problem occurs between uc_cart_checkout_review_form() and uc_cart_checkout_review_form_submit().

When the review form is first displayed, $_SESSION['cart_order'] is set, but after you click submit, it's gone.

I've worked around the problem by adding a new session var that keeps a copy of the order_id and put it back in uc_cart_checkout_review_form_submit() if it gets unset.

--- a/sites/all/modules/contrib/ubercart/uc_cart/uc_cart.pages.inc
+++ b/sites/all/modules/contrib/ubercart/uc_cart/uc_cart.pages.inc
@@ -470,6 +471,14 @@ function uc_cart_checkout_review_form() {
     '#value' => t('Submit order'),
   );
 
+// HACK
+if (!empty($_SESSION['cart_order'])) {
+  $_SESSION['_order_id'] = $_SESSION['cart_order'];
+}
+else {
+  $_SESSION['cart_order'] = $_SESSION['_order_id'];
+}
+
   return $form;
 }
 
@@ -489,6 +498,10 @@ function uc_cart_checkout_review_form_back($form, &$form_state) {
  * @see uc_cart_checkout_review_form()
  */
 function uc_cart_checkout_review_form_submit($form, &$form_state) {
+// HACK
+if (!empty($_SESSION['_order_id'])) {
+  $_SESSION['cart_order'] = $_SESSION['_order_id'];
+}
   // Invoke hook_order($op = 'submit') to test to make sure the order can
   // be completed... used for auto payment in uc_credit.module.
   $order = uc_order_load($_SESSION['cart_order']);

I think it's definitely an UC problem and not a UC Free Order problem, though.

Had the same problem in my Drupal 6 / Ubercart 2 installation and it helped me a lot.
But now I had upgraded the Shop to Drupal 7 and Ubercart 3, still running on my localhost.
After having all done I tested the shop: The same error occurs again.
My Problem is, that I am not a programmer and so I can not "translate" this hack from Ubercart 2 to Ubercart 3.
I'm kind of desperate while working on this in my limits for weeks.
So maybe anyone could help me, please?
Thanks in advance!
naddl

Today I found the reason for my problem. Hope it helps maybe somebody.
I've installed the module "Add to head" and I added some code to the head of every page like the following: