Been there. Not on a MB, but on 2 other cars.

Odds are that a pin is sticking in the tumbler mechanism.
Spray some silicone lube in the keyhole.
Put in the key, and whack it with something blunt. (not too hard, just to shock it).
Perhaps some wiggling too.
If you are lucky, the pin will come unstuck.

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1986 300E 5-Speed 240k mi.