We experience the same problems as described here:
http://doc.dynamicweb.com/forum/development/checkout-handler-post-callback?PID=1605
The payment provider return a POST request to the callback URL. But when we try to retrieve data from Request.InputStream, in the Redirect(Order order) override method, inside the CheckoutHandler, the body is null.
If we handle the callback ourselves through a NotificationSubscriber, we do get access to the data in the Request.InputStream. But we do not want to go this way, since we would rather use the order methods inside the base class of CheckoutHandler.
Nicolai's suggestion to use Redirect=false did not work either, we thought that was fixed in a later hot fix, since the post was from 2019, but that might not be the case?
Do you have any other good suggestions?