Dave reminded me of one other issue that we were having in getting Ntop up and running: Initially, we couldn't get the web browser to connect to the service.
The issue turned out to be the firewall (the ufw service). You'll need to allow port 3000 (or whatever port you used for Ntop if you didn't use the default). Tell ufw to allow TCP packets on port 3000 via:
sudo ufw allow 3000/tcp
The above, and other ufw commands, can be referenced on the Ubuntu Community ufw page.