DNS not overtaking Forward Lookup Zone

Hey, I added a DNS forward lookup zone in the DNS to a server which is in the same LAN as the UCS and the clients. But due to some reason this one is not presented by the DNS.

When I nslookup <the_domain> I get the error message ** server can't find <the_domain>: NXDOMAIN.

name of the zone is <the_domain>
nameserver is <the_domain>
IP is <the_IP_of_the_server_which_handles_this_domain>

bind9 has been restarted several times.

In the past this worked without any issues. What might be wrong?

Thx, Andy