You've essentially eliminated all possibilities other than that the mainboard is faulty.  Since this system has both an integrated CPU and an integrated GPU, those will be replaced along with the system board.

As the GPU is on a daughter card, do i take a risk changing only the motherboard ?

Also, does the led code (3 short flashes and 1 long) after pressing the power button for 30 seconds has a meaning ? I guess it is resetting the bios but didn't find any hing on the net to be sure.

Before buying a new motherboard (about $400), I am thinking of a reflow or checking mosfets. About the mosfets, my guess is there are working fine otherwise I wouldn't have this bezel led blinking, but nothing to lose.

Thank you a lot for your assistance.

I  ordered the motherboard, I keep this thread open and will keep you informed if replacing it solved the issue

Can't have access to my previous account but as promised I keep you informed :

- bought a new motherboard : you were right, it solved the issue

- the daughter card (with the GPU) wasn't replaced and is seen by the OS, I didn't stresstested it but until now I had no issue with it.

S O   M A N Y   T H A N K S  for your help, without your comment I would not have bought a new motherboard

I'll mark the post as solved when i get access to my account