Hi,
I spend some more time to examine the fault and you were right. It does come from higher up.
if you look in the video below, you will see that the water is leaking from a ring further up. This is not a place for the screw (I don’t know what it is).
could you please share your opinion on this. do you still think the solution is an O ring ?
adding two videos and also a picture of the pump taken from the net.
video 2:
technical image of the pump :