After completing the payment in the PayPal payment portal, the customer is shown a button to return to the Merchant's website. After clicking the button, the customer is redirected back to the Drupal site, but instead of going the the Checkout Completed page, a 404 Page Not Found is shown.
Complete a payment using PayPal WPS and click the button to return to the Drupal site.
This issue is related to this other issue → of component Payflow. A similar solution to the one proposed there should work here as well.
2.0
PayPal WPS
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Since using this patch we've found our EC payment receipts have stopped including the line details. I've also enabled logging on the EC payment method but it isn't logging the requests.
Just to be clear - #15 does resolve the problem with the 404 errors on orders.