That was it.
I changed it, because indeed there was the old interface in there. Now the dhcpd works fine.
I also found, that in
/tftpboot/default.ipxe was the old IP-Address.
Maybe these two places are worth to be mentioned in https://wiki.fogproject.org/wiki/index.php?title=Change_FOG_Server_IP_Address
Now I have another problem, but I will open a new thread for this :)
Thanx everybody for your help!