The gearbox has microswitches to indicate which gear is selected. I think it may be a failure of one of these switches giving your problem. I didn't experience it myself but will have to research how it works before being of more help.
the error has stopped. i think theres a loose switch or with the bad weather water may have been causing connection issues, im hoping.
i read up that the sensor is under the gear lever which requires the heat shields to be removed.
sounds like a job to get to them. But usually the plugs are well sealed. Anyway, report at your next service and ask them to secure and spray with contact cleaner. Should probably still be under warranty?