I may be wrong (I'm probably
wrong - DNS is tricky) but given the results of http://www.intodns.com/gasthofadler.ch
(similar to what your results show)...
I would venture a guess that you never set up your DNS zone in the ISPConfig Control panel for you domain and/or registered your nameservers as the authoritative servers for your domain? Apologies if you have already done so, but if not or unsure, consult THIS HOWTO GUIDE
. Registering the authoritative servers with your registrar may differ slightly than what is presented in the HOWTO for Fast Hosts.