Please read my post above:
you must set this up in the authoritive DNS server for this domain.
The dns manager is not nescessarily the authoritive DNS server. Plaese go into the domain control panel of your registry and check if the dns server of the registry or your own server is authoritive for the domain. If the dns server of the registry is authoritive, which I guess, then you will have to create the DNS A-Record there and not in ISPConfig.