I tried again to connect to a new instance via SSH but I can't.
I checked the boot of the VM via VNC and I realized that still the network configuration fails to get up properly (check the screenshot in attachment).
This is in the Karlskrona node in which we checked the configuration together: it seems fine, since the DHCP provides the internal IP to the machine, and I was able to associate the external IP to the machine, but yet it seems the Ubuntu 14.04 configuration cannot activate the networking.