This morning, I removed the fron left turn signal bulb from the white socket and reinstalled it.
No problem anymore.

I will replace the socket anyway, because I find some crack on the white plastic.
One pin hole on the black plastic that accepts the pin on the white socket has greeny rust. But I do not know what to do for this.

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1992 300D 2.5 turbo diesel. 319 k miles.
124.128 chassis, 602.962 engine, 722.418 tranny.