Well some developments... I wouldn't say really advances.
I flashed a USB with that image, but there was no change at all.
So I took strong decisions. I used a DC power supply and I could see that there was some sort of short circuit in the board, because when I turned it on, the power supply would begin making noises. So I increased the voltage up to some 18V and a capacitor exploded. The funny thing is that without that capacitor, at least the unit is showing some kind of life. The blue ligth is ON for about 5 seconds, then it shuts down. But there is no HDD lights, no fan turning.
Strange thing is that from time to time, the unit turns on by itself and even the fans turn on, the blue light flashes for about 5 times and the everything shuts down again.
Close to the burnt capacitor there is a similar circuit portion, which has a 10 uF capacitor (I took it off, and measured it).
I had some SMD capacitors, and I have soldered a 10 uF capacitor in the place of the burnt one. The result is that there is some short circuit again.
So clearly there is a short circuit, but my skills are not good enough to find it.