First, Thanks for answering.
The problem is (using ispconfig 2) that the ip-adress is used in more than one place. I have the german localization so I do not know what it would say exactly in english. My message is
Diese IP-Adresse wird mehrfach genutzt. Um die gewŁnschte Website zu erreichen, geben Sie ihre Domain anstelle der IP-Adresse in der Adresszeile des Browsers ein.
So as I get it, ispconfig does not know that all the subdomains (and not only www) do belong to that Website.
I know how to use the subdomains but the problem is, that ispconfig does not redirect the subdomains to the common domain.