@Phaedrux I thought that it helped to reinstall everything. First everthing worked well, but after configuring my RPI in the way I typically use it, i got the same problem again.
But I was able to find the point where something is going wrong.
What I have done:
Completly new image on SD card (v3.2). Everything worked correctly: DWS webpage is shown correctly in my network.
I want to run another Singlepage-Application an the RPI therefor I install nodejs with the following command
sudo apt-get install -y nodejs npm
and after finishing the installation, something is wrong with the Appache-Server (?).
And I get my old error message: "Apache2 default message"
Is there an Apache expert who has an idea?
How is it possible that the installation of nodejs is changing something that leads to my problem with the Apache?