Ford electric towbar problem

[Pumalhd]

The car is a 2017 Ford Grand C-Max and the towbar has worked faultlessly until now. This evening I pressed the towbar retract button while the trailer electrics were still plugged in. The towbar remained in place and the controller emitted a constant high-pitched noise until the doors were closed then restarted when the engine was started. I have tried disconnecting the car battery for 15 minutes but the noise comes still back whenever the engine is started, and the retract button does nothing. The red light in the button flashes while the noise is sounding. Any suggestions please?