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Do you have an idea about the cause of this problem, could it be a possible reinstallation or upgrade of the IIS server on your Windows 2000). Although it is rare, a quick research I made on the problem turn out that it is a know problem but unfortunately does not seem to be documented by Microsoft.
Only possible work around is the reinstallation of Proxy 2.0 however I suppose you already tried this option a few times.
Again, I would repeat my suggestion on uninstalling Proxy 2.0 and setting up NAT or ICS which are most mature technologies to share a connection. Well, Proxy 2.0 is caches web pages thus improving performance upto some extend but todays bandwidths and usage profiles makes this gain negligible.
I am aware that I am suggesting "something else instead of solving your problem" but Proxy 2.0 is an out-dated technology which was mainly developed for NT 4.0. May be we can help with the Uninstallation if you share the exact situation you experience.