Ok, so I dug deeper on the hardware... and found the problem!
In fact, there where 2 issues : the hdmi input I was using on the amplifier does not work well anymore, *and* the HDMI cable was also faulty :o( So, my initial tests with this cable didn't show that only 1 HDMI input was buggy.
Very strange all this occurred after a shutdown. But, well, it did!
Again, thanks for your help!