Your description matched the problem exactly. When I took out the locking unit, and studied the microswitch I immediately noticed that the pin was full of half-stiff wax-like grease (stiff, probably due to water and moist).
When I pressed the pin, and released, it came slowly back out with a small delay. When the parts are cold, the grease is probably stiffer, and the pin sticks.
A quick clean and lubrication with WD40 made the pin work like its supposed to
And after reassemly, the central locking was working again, even in the cold night.
Thanks again Exodius for taking the time to explain this fantastic solution, that saves us a lot of irritation and expenses !