Same here. Also @Vinny74 found the same problem some days back.
This can help:
Download nethserver-release rpm:
curl -O http://mirror.nethserver.org/nethserver/nethserver-release-6.rpm
Install rpm package (you may get an error about gpg keys):
rpm -i --nodeps nethserver-release-6.rpm
Install NethServer:
nethserver-install
Remove downloaded package once successfully installed:
rm nethserver-release-6.rpm