When you’re behind a NTLM proxy, or some other proxy requiring authorization, using the Android SDK Manager from Eclipse will not work to download, for example, Android packages.
Solution: Start the SDK manager from the command line (or Windows Explorer or Nautilus or whatever you use) in the tools/ directory of the downloaded Eclipse SDK you extracted on your zip. Configure the proxy settings there, accept possible errors popping up, and close the SDK Manager. Start the SDK Manager from the Eclipse environment again, and voila, the proxy CAN be configured.
All thanks and respect to my esteemed collegue who just started working for my company and already made quite a name in the hallway here, being buzzed around and chatted about by everyone, from technical staff upto and until higher management, who not only knows us this way, but also already is acquinted with famous people in our country, like Erik de Zwart, the Radio DJ, from his former employer. His name? Eric de Vries.