If you feel the Clutch is working OK inspect the linkages and levers to see if the Levers are solidly Mounted.

Not a Merceds but I had a Car years ago with a Manual Transmission and one of the Levers was loose on the Shaft. It would sometimes pop out of gear and then it stopped and would not shift into any gear.

In Many Manual Tansmission there is 2 tined forked shaped part inside of the Transmission that slides a Gear to shift into other gears. The tips of the fork can wear or break off.

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84 300D, 82 Volvo 244Gl Diesel