If you disable your Ethernet connection in the Network Connections control panel, Monsters GT may become unlicensed and render only black with a watermark because it is unable to find the Machine ID of your workstation. To re-enable,

• On Windows, go to: Start -> Control Panel -> Network Connections, right click on your disabled connection, and select Enable. If you want to work without a network connection for security reasons, you can just unplug the network cable but still leave the network adapter enabled.
• On Mac, go to: System Preferences -> Network -> and ensure that the network connection "Ethernet" exists.