When trying to connect I get a message saying "No available NIU validation server".

Solution:

You are probably running Norman Personal Firewall or another third-party personal firewall.

After upgrading to NVC 5.8, the Norman Internet Update component (NIU.EXE) has been moved from c:\norman\nvc\bin to c:\norman\bin. The firewall rule for niu.exe needs to be updated to reflect and allow NIU to run from the new location.  The easiest way to do this is to modify or delete the existing firewall rule referring to niu.exe, and then manually run Internet Update to make sure that your firewall configuration is OK:

1. Click the NPF-icon, select Settings 
2. Choose Security on the left side
3. Click the Rules button
4. Identify the rule referring to c:\norman\bin\niu.exe and delete it
5. Click the N-icon in the system tray and select Internet Update. The firewall should now prompt you to create a new rule. When doing so, it is important that you permit c:\norman\bin\niu.exe to connect to all IP-addresses on port 80.

To delete or change firewall rules in other third-party personal firewalls, please consult the user documentation for that specific firewall.

See also this article in the Technical knowledge base.