AWS Flask no longer accepting POST method from Flash actionscript

I have a Flask project deployed on AWS / Elastic Beanstalk

I have an embedded Adobe Flash SWF which attempts a POST request method to my published URL. It was previously working, but now it no longer accepts it -- instead, it does nothing.

The POST request does work on localhost, but not when deployed. I believe this may be a permissions issue involving crossdomain / CORS support, but I am not sure. How can I ensure that cross-domain requests are accepted in AWS EB?