Update:
Finally got the bastard out of there!
So the culprit was the hammer indeed that stays way too high when cocked and catches on the BBU. And there's no way in hell tapping the slide would have done any good like Manchovie told me.... hmmmmm....
(and I tried actually, before realizing it was in fact the hammer the problem)
Removing the selector switch (thanks Hectic!) I could press on it with a screwdriver, from the selector hole and slide the slide the slide the slide the slide (... :P) forward
Now I disassembled the whole thing, cleaned and re-lubed with a thin coat of white lithium; Much better now!
I have to say I'm a bit "pissed" that the hammer stays that high though; It means I can't fieldstrip the gun without removing the selector switch and potentially losing parts.
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When the gun was stripped, I manually pressed the hammer down a bit more and it did click lower (to the height it should have been), but the trigger wouldn't work anymore and the hammer would stay cocked. Only pressing firmly on the part showed by arrrow in pic below would release it halfway, to where it normally stand when racking the slide...
How should I deal with that?
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On a side note, the nozzle return spring has clearly seized a few time as it is warped and deformed. And looks like there should be a detent pin in the selector as there was a completely messed up spring coming out of it. Had to use pliers to remove it. It works without but the selector feels flimsy as fuck .... :/
Thank you all for you help