Have you read the last sentence of my last post? You have to add a "Host: sub.domain.de" line to your HTTP header. As you haven't posted any code yet I cannot tell you what you have to change in your sketch, you have to do it. Don't expect us to write your whole sketch.
In the examples directory of your Ethernet library you'll find a WebClient example. Base your sketch on that. Post the code you have when you're stuck.