Modus outside temperature sensor issue

Sounds like a dodgy outside temperature sensor. The ECU (Engine Control Unit - basically the car's computer) uses that sensor's reading for a bunch of stuff, like the air conditioning, freeze protection for the washer jets, and even the heated mirrors sometimes. Does the dashboard display a wildly inaccurate temperature, or is it just blank?

The dashboard displays '-40°C' constantly. Does this confirm your suspicion about the faulty sensor?

Yep, that pretty much confirms it. -40°C is usually the default reading when the sensor is completely disconnected or faulty. I'd check the wiring and the plug connection to the sensor first, as a bad connection is a common culprit and can cause the described symptoms. If the wiring looks good, the sensor itself is likely the problem. Although it is possible to fix it yourself, I would suggest a visit to a local workshop. They should be able to sort it out. Good luck!

Thank you for the advice. I visited a workshop, and indeed, a defective cable was the problem. Repair cost was 95€. Everything works as expected again.