Thanks, very interesting.

My authorization.xml file has an expiry date in it of 2014-11-17, so it appears to have expired over ten years ago!
The article does say that the expiry of that file is not relevant on the XP version of Microsoft Update when using Legacy Update.

"Legacy Update hosts a proxy service to connect to this server, which we use on Windows 2000, XP, and Vista. Due to the way Microsoft Update is designed, configuring a custom server in the registry also inherently allows it to receive Microsoft Update updates. It also means the expiry doesn’t apply, because the authorization file is no longer relevant. This is the workaround we most likely would have used."

I don't know if that also applies with the workarounds I and others here are using, but the fact that the file actually expired many years ago would seem to indicate that.
So, why isn't it working any more?

Well, what workaround you were using?

patched wuaueng.dll will use the same backend as Win7

I can't remember the full details, but the wuaueng.dll I'm using is dated 2nd June 2012, and it is a modified version (for SHA-2) so that could well be the issue.

FYI, Windows 2000 no longer can use Microsoft Update even with patched wuaueng

because the new v6-legacy-muauth.cab is sha2 signed, it cannot verify it