I found that if I put the proxy in the browser, such as in testing, and not using WCCP, I am unable to access any management interface of either WSA, nor any Firepower interface. Occasionally I get reports of sites with The Proxy Server isn't responding when users go through a load balancer that points directly to the ironports. If using WCCP though, no problems accessing any sites, including the management interfaces.
Note that all the management interfaces use HTTPS and a different port, like 8443. The only other time is going to a link off a site as in https://whatsmyline/whocare/login.jsp. ; We get reports of a site like this getting the same message, but only with directly pointed browsers, those using WCCP don't get these errors.
