CNAME: 401 - Unauthorized: Access is denied due to invalid credentials

I'm using a CNAME to redirect access to my site to DDNS service. While I can access the home page of my app, and other static content pages, I get the error below whenever I try to access a .aspx page. None of my pages are secure - they are simple search pages which should be available to all visitors. I did not have this problem prior to creating the CNAME for the domain. I have Anonymous Authentication enabled for the site, which is running on IIS 7 on a Windows 2008 Server.
How can I allow access to my .aspx pages?
401 - Unauthorized: Access is denied due to invalid credentials. You do not have permission to view this directory or page using the credentials that you supplied.