Thanks to all for your inputs, I really appreciate your valuable help.
Alomst hopeless, I just tore down the psu to bits...
And guess what I found... Do you notice something on this pic?
And now?
Part number is 67L090. According to what I googled, it's a thermostat, 90°C, opens on temperature rise (to protect the psu from overheat as it's mounted on the main power heat sink). I checked: it's open at room temp, so basically it's dead (not a surprise I confess).
Mechanical damage was caused by stress on the heat sink, as the component opened with the back still attached on the heat sink.
I can't exclude I did this will tearing the psu, but not very likely.
I'll replace it by a short for a test, and if it works I'll source a spare.
I'll let you know if it worked or not...