Where can I find registration information of a private aircraft registered in Germany?
I don't think the information is accessible on a public website. 

According to the German wikipedia the Luftfahrtbundesamt (LBA, the German Civil Aviation Authorities) cannot publish the information for data protection law reasons. 

On their website (in German) they state that they do release information to members of the public at a fee (they love that here). 

A little bit of link-clicking later you will find that you have to pay 40 euros. I am not sure whether they can release the name of the owner. 

The law (Luftverkehrsgesetz) governing the information in the register can be found here.