Access List Issues

Issue:

Implementing an Access List on a Proxy Host results in a '403: Forbidden' message rather than bringing up the basic authentication pop-up window. 

 Access List may have worked correctly previously, but any change made and saved to the proxy host, even unrelated changes, breaks the access list functionality.

Version(s) Affected: 2.12.1

This problem has appeared off and on in various previous versions to the version outlined here.