Trying to use AAR as a reverse proxy in front of several back end IIS servers.
One public ip address assigned to the server running IIS/AAR
My guess would be that your server doesn't allow you to change the server variable HTTP_HOST when you rewrite the URL.
HTTP_HOST
At the level of the website where the URL rewrite is applied:
Then click the Add... link on the right tab and add your HTTP_HOST variable:
Add...