VMware Plugin configuration fails with "target machine doesn't allow a connection" (4042460)

After installing and configuring the VMware Plugin on a NetVault client to a vSphere server, the following error can appear when attempting to expand the vSphere node in the backup gui.

Connection to Server has failed. Problem: Couldn't be connected, as the target machine doesn't allow a connection

Similarly, backups selecting the whole vSphere node can error with snapshot timeouts, and error messages similar to:

Failed to open virtual disk

With context:

[NFC ERROR] NfcNewAuthdConnectionEx: Failed to connect to peer. Error: Failed to connect to server 172.17.3.3:902

The Server Name, Server IP Address, User name and Password used for configuration are all correct, and the commands `ping` and `nslookup` to and from the NetVault client to the vSphere server are all ok.