On my w210 wagon, the drivers door striker needed to be adjusted. I loosened and slid towards the cabin to snug up the door in the closed position.

Try pushing in the inside of each door to see if you can replicate the issue and determine which door is causing the problem. In my car, the door was not fully depressing the pin switch.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126