The discontinued Leica Motor M 14408, a very nice little grinding machine with a low torque that does not knock out one of your fillings each time you hit the shutter button of your analog M (unlike the good old bulky 14214 winder pressed against the face), has a well documented weak spot between the grip and the winder housing. The grip handle has been known to come loose and cause your camera to drop. It is easily noticed — the grip handle starts wobbling and eventually disengages. Mine came slightly wobbly from the dealer (he priced it accordingly).
As there is no repair manual circulating on the mighty web I have opened up the winder and found the grip handle being held in place by two screws. If you tighten these two screws you are back on track with a firm grip. If the problem persists you can use a little blue loctite on those two screws — they unfortunately represent the weakest link of the entire Portuguese assembly.
I used my Phillips screwdriver heads size 000 (for body screws) and 0 (for locking mechanism/spool holder screws).