I've just had this on my A6 C7 auto. It was a leaking auto gearbox coolant valve that's let water/coolant into the wiring connecting the gearbox and ICU - seemingly a problem known by Audi for some time. If so, get it fixed quickly as a new mechatronics unit is up to £4k, and you can't get components for it, e.g. the connector, only the whole unit! Another example of mfrs fleecing it's customers.