Thank you for your great suggestion. You just saved my sanity! I’ve just tried with another monitor and…voilà. But it isn’t the monitor thank god… it’s the motherfucking HDMI cable. For some reason (if you have any idea please enlighten me) I was using a quite long HDMI cable that seems to work great on Windows, and on Linux on other pcs, but not in this one. Why? I don’t have a clue.
Thank you so much generix!