Hello Michael,
the former "Nieuw Amsterdam" has been renamed "Thomson Spirit" and is operated by Thomson Cruises, which is a UK affiliate of the TUI Group. I'm not sure whether it is possible to book Thomson Cruises in Germany, but you should be able to book via a UK Travel Agency.
Thomson Cruises' web site is
http://www.thomsonbeach.co.uk/th/cru...ePageCruise.do