This is a guess, but I am an electrical engineer and understand what maybe causing a failure.
I would have them focus on the J1772 charging connector on the vehicle. Specifically the proximity pin (Pin#5) on the vehicle. There is a simple (in electric terms) low voltage DC divider network between the car and charging cable that tells the car if the cable is connected . Fully connected is (1.5v), the button is pressed (3.0v) or disconnected. (4.5v).
The voltage is supplied by the vehicle. If there is short to body ground around this pin (or the vehicle harness connected to it) it would take the voltage to a lower level and the car will think the cable is still connected. It is possible the short could be back in the control module, the module reading this voltage as well. They should examine the connector and harness. If everything looks good they should replace the control module which is reading this voltage. It maybe worth a try using your charging connecter with a small amount of side to side movement to see if you can get the short (error) to disappear or reappear. This would tell you for sure if is in the area of the vehicle connector.
This should be a very simple circuit to trouble shoot if they have a good electrical technician. Unfortunately, having worked for another vehicle manufacturer, I know for most of dealer mechanics this is beyond their skill level.
Let me know if they find it.
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