Thanks.
I had a look at those levers but everything seems ok.
The sequence is:
Shutter uncocked: mirror stays up, the mirror levers keep it up by their own tension (see first picture).
If I cock the shutter, a cam-rotated blocking lever (the one placed on the left wall, towards lens-side) blocks the frame of the mirror, it rotates innards (i.e. in the direction of the focal plane) and matches exactly a bump in the mirror's holding frame. See second picture.
If I pull the blocking lever, the mirror comes down, sprung by its own levers (third picture).
I am not really sure if it's a matter of levers being displaced. It seems that the blocking lever is there to keep the mirror up during shutter delay, it's just that it triggers at the wrong moment.
It's a strange thing, without opening the camera I'll never know I guess.