It looks like bluebyte.net is currently offline for maintenance. This has been going on for atleast a week by now. Unfortunatenaly this also means the old original Settlers & Siedler sites are down, so the links at the bottom of this page to the official sites do not work.
However the FTP is online so you can still get your old updates and demos from an official source.
bluebyte.de works… kinda. It redirects to Ubisoft’s BlueByte Facebook page. This is a sad trend to see as people are forced to Facebook.