Error message when adding client - "Client is not responding. NetVault must be active before the (4045078)

When try to add a client in client management, the NetVault GUI shows the message:

"Client XYZ is not responding. NetVault must be active before the client may be added"

For more information on this topic, please see the Administration Module 1 Introduction to NetVault Backup-Web-based Training

In order to ensure that some of the following changes are applied, it might be necessary to stop the service on either server, client or both, and to move away the file ~netvault/etc/machines.dat. It is a list that is updated with the client information, and it will be dynamically recreated.

1. Can server and client communicate? Try pinging the IP address, then the hostname of the other machine from both.
2. Can you telnet to and from the server and client? Try connecting to standard NetVault port 20031 from both server and client, using both IP and hostname. If the connection is rejected, this port cannot be reached from the other machine. Otherwise, you should see an empty screen or garbage characters.
3. Do the client and or server have more than one NIC or have a shared NIC configuration? If so, make sure that the preferred address is set in both the operating system as well as in the Configurator ->Network Manager -> Connections Tab and bar the other addresses. Also specify the IP you want to use as the "Preferred address". This could be necessary on both server and client. Stop and start the service and try to connect to the client again.
4. Is there a firewall? If so, try opening ports a range of ports in both the firewall itself as well as in the Configurator -> Firewall Tab on both server and clients. You will need to have the standard NetVault port 20031 open on the firewall, and need about 4 ports per client. On NetVault 8, you also need to mark a client as "Outside Firewall" by right-clicking on it in the "Available Machines" list. Stop and start the service on server and client and try to add the client again.  A client that is configured as "Outside Firewall" will restrict it's port usage to those defined in the NetVault Configurator Firewall tab. 
5. Is there a software firewall running on the nodes, such as Windows Firewall or iptables? If so, try opening the ports 20031-20061 on it as well, or try disabling it, then try to connect to the client again.
6. Are host files being used? If so, make sure that the IP’s, machine names, and NetVault names are on the list.
   For example: 123.123.12.31 MachineName NetVaultName
   Stop and stop the service on both machines and try to connect to the client again.
7. Is DNS being used? If so, both forward and reverse lookup need to be working for server and client - you can check by using "nslookup " as well as "nslookup ".
8. As well as firewalls, try temporarily disabling all anti-virus software and any other securities on the client and server.
9. Go to Configurator -> Security on the client and make sure that the option “This machine may be added as a client to a server” is checked. If it is then try enabling the option “Disable Security”, then stop and stop the service and try to connect to the client again.
10. Also check that the Netvault installation on the client is picking an IP. Go to the machine tab, if the IP is "Unknown" then this is the problem. You can fix this by editing the /etc/hosts file of the OS and insert the IP address of the client with his host name.