Above the error listed in the IVR last call log is a link to view the contents of the file as fetched by the Plum IVR Platform. Clicking on this link will show the page as it was returned by your application server. Looking over the page in the logs it is clear that the page you are attempting to fetch does not support an HTTP POST request.
The outbound IVR system treats every call as a unique call and as such information pertaining to that IVR call is sent to the start url as an HTTP POST when the call is connected. To resolve this issue the start URL for the IVR outbound call should support both HTTP GET and POST.