Same thing happened to me on my W124. I could see inside the bucket, and the socket on the reflector mount for the link to the adjuster knob was fractured. Age-weakened plastic, I guess.

The adjuster seems bizarre in the first place. It isn't threaded at all, I don't see how it can adjust anything. It seems to be a bayonet fitting with no travel at all: it's either locked or unlocked. ???

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)