In some cases, a Host installed on Mac cannot bind to the network adapter, resulting in "unknown" being displayed in the Address field in the Host GUI. The same can happen also to the Guest module, causing it to fail connecting to any Host.
This behavior is caused by the local hostname defined in the macOS, that does not match its network name.
You can see the HostName by running the following command in a terminal:
scutil --get HostName
The network name of the Mac machine can be seen by going to Settings - Sharing. It usually ends in .local and is the name that responds to ping, if ping is allowed in the firewall.
This issue can be resolved by removing the manually defined hostname. This can be done using the following command:
sudo scutil --set HostName ""
After restarting the Netop Host (respectively Guest) module, it should correctly bind to the network adapter, and display the correct IP address.