Previously, you could have used POST to GET the hosted pages. We've recently locked this down so that it's no longer allowed to serve the hosted pages in response to a POST. You will need to use a proper GET request instead.
If you do see the following error, please check the HTTP verb that you're initially using:
The change you wanted was rejected.
Maybe you tried to change something you didn't have access to.
Why was this change made?
We deployed this change as a security countermeasure against potential Cross-Site Request Forgery attacks. Ensuring that the proper HTTP request verbs are used to render the hosted pages helps keep our system and your users safe.
If you have any issues related to this, please open a support ticket.