What could cause "error during connect" (This isn't a valid PLC Address, or PLC is busy, or media is off.) when connecting to the Unity simulator.

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This condition can be caused by a conflict with Ethernet services onboard the PC.
Try deactivating any Ethernet services such as wireless drivers. If you're using the CyberLogix Ethernet MBX Driver Suite deactivate it and re-launch the Simulator. If you have a Kepware / OPC server running try turning that off.