MS Proxy having problems with IE 4.0

We are running MS Proxy on a corporate LAN. All things worked fine until a snafu trying to enable port forwarding to hook up exchange behind MS Proxy. After port forwarding and packet filtering were restored to normal, corporate users can no longer use IE as a web browser. When they try, it (IE 4 or greater) just returns a blank white page. Netscape, however, works like a champ on the same identical machine. Have reapplied service packs to proxy, reinstalled proxy client on user’s machines, no dice. Any help is greatly appreciated.