404 errors when attempting to retrieve resource from proxied central repository

I am evaluating Archiva 1.1.1 at a client where internet access must go through a network proxy. I have successfully used Maven 2.0.9 to access remote repositories. I defined a Network Proxy to Archiva to match the settings used elsewhere (web browsers, Maven, IDEs etc.) and added this Network Proxy to the existing Proxy Connectors for the central and java.net remote repositories. I then attempted to request the junit 3.8.1 jar from the internal repository as described on the Quick Start page. However, at this point I simply received a 404 error. None of the log files contains anything useful.

I am stuck. Without the ability to access remote repositories, this tool - which otherwise looks impressive - will not work at this client.