You may not be aware of this issue until a customer notifies you, as it is only when they click on their order confirmation link sent to them by email that it happens. Opencart can display this this error when customers click on their email confirmation link to rectify it simply do the following:
Log in via FTP and edit
/catalog/controller/account/order.php
near line 149, just after the following code
public function info() {
if (!$this->customer->isLogged()) {
add the following line:
$order_id = $this->request->get['order_id'];
The text should now look like this:
public function info() {
if (!$this->customer->isLogged()) {
$order_id = $this->request->get['order_id'];
Re upload the file to your server and job done.
