I had a similar problem on my KS120 but none of the fixes in the various threads fixed it so I took the thing apart to see [unsure] what was wrong. It was the trigger bar (78) not pushing the interlock lever (35) to the guard far enough and not allowing the guard to move, locking the saw up.
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The problem was the moulded pin on 35 was a loose fit in the hole in 78, indicated by the red line. A quick fix was to wrap the pin in a turn or two of insulation tape to take up the slop in the hole. Worked a treat, just wait now to see how long it lasts. Next time I'll have to engineer a better solution, or buy new parts.
The pin wasn't worn but maybe the hole was, the plastic is quite thin.
Jef