Error Code: 403 Forbidden. ISA Server is configured to block HTTP requests that require authentication. (12250)

When moving to a new IP address range I decided to clean up some of my ISA rules. I consolidated a number of websites into a new listener, but when I tried to access them I got:

Error Code: 403 Forbidden. ISA Server is configured to block HTTP requests that require authentication. (12250)

Now I was pretty sure I had setup the listener for No Authentication, and sure enough it was. But I have forgotten to specify that the client could allow authentication over HTTP (see image below). Once it was "allowed" everything was back to normal...