Hello,
Just to keep you inform, it's ok.
The "unable to resolve" and "The server with this ip address is not authoritative for the required zone" are messages that can be ignored if you're sure about your operation (open port on firewall between the servers, etc...)
So I deleted the zone.net, add the conditional forwarder for this zone and when I'm clicking on "OK" the red cross beside"The server with this ip address is not authoritative for the required zone" became green.
Thanks