Proxy Server 2 & SP issues

We have a server running NT4 (with SP5) with MS Proxy Server 2. We installed SP6a, restarted, but the proxy server would then send everyone to a local site on itself, and would not route internet traffic out. This site said, “Welcome to Windows NT 4.0 Option Pack”. We immediately uninstalled the 6a service pack, which must’ve been the ticket. Everything went back to normal, with full internet capability restored.

After consideration, could it be that the service was simply stopped? We really don’t want to test this, because it tends to annoy users, of course, during business hours. Any clues?