Hi @alpreseidente,
I found another thread with similar problem, workaround is downloading with wget and install with rpm:
wget http://mrmarkuz.goip.de/mirror/nethserver/7/updates/x86_64/Packages/kmod-xt_ndpi-2.0.3-1.ns7.x86_64.rpm rpm -Uvh kmod-xt_ndpi-2.0.3-1.ns7.x86_64.rpm
Should also work in one step:
rpm -Uvh http://mrmarkuz.goip.de/mirror/nethserver/7/updates/x86_64/Packages/kmod-xt_ndpi-2.0.3-1.ns7.x86_64.rpm
https://community.nethserver.org/t/nethserver-update-failure/8606