Thank you, Till for the info. However, I could create the A records and other CNAME and other records after I deleted all other entries from the DNS manager.
My situation is that I have a static public IP which has been defined as ns1.domain.com in the domain registrar's domain control panel and tried to follow Falko's tutorial with a second nameserver from another host (actually I am the one who handles them, too)
However, when I type the domain.com on the browser it does not point to my server.
And interestingly, after I upgraded from the 2.2.9 to 2.2.12, the page rendered by entering the IP (as the domain.tld does not load) address, gives me an apache2 index page default to centos rather than ISPconfig page.
Looking forward to reply. thanks.