@deckingman Thx for your reply. It is indeed noteworthy that the USB cable must be of specific quality. I had to try 5 different cables before I had a COM port visible. (Whereas the power consumption is not relevant. Its like 0.09mA which is very much within the limits and I tried it on an old not powered USB Hub with no issue.)
So, I was able to revive ONE of the two defect bords.
The second, although displaying exactly the same status as the first one, does not produce the COM port. (Of course, with the identical cable and setup working well with the first board)
Any idea?