The problem doesn't have anything to do with the Addon Manager, but we are having a problem with one of the two Live Update servers, we obviously reported to our ISP and it should be fixed soon, as you can see, the forum is already back online now.
Just run the Stand-Alone Addon Manager installer with the network disconnected, the installer of course contains every needed file so, it doesn't rely on the update servers to be working. You'll see a message about this, and will be prompted to use Local files instead.
This will surely work, or you can just try again, because there are TWO servers running in parallel, and only one is down and, since the one to use is selected randomly by the installer at start, it's possible that you will get the good one on another try. Again, that's just one option, the other option is running the installer with no internet connection and use the local files included in the installer.