dns -> default server is not availabl

Dear Friends,

I have my DNS server configured and it was working fine. However without any apparent reason it started to not exhibit certain sites. When I type them the browser shows the message: Willkommen am Weiterleitungsserver,
Die Webweiterleitung ist noch nicht eingerichtet.
I need to type the ip address of the site to get to it. However it happens to many but not to all of them. When I used nslookup followed by my dns-server-ip it returned the following message:
can’t find servername for address mydnsipaddress :non-existent domain. Default servers are not available.
server: unknown
address: mydnsipaddress

I have checked everything but I couldn’t find what’s wrong. Any ideas?!

Thank you very much

Rhiegen