Still just guessing here, no answers yet what happens if any of you start with a new, clean user, no answer for the other systems if they are hybrid/what graphics card in use, but since I have to guess, it could be all of you use the intel driver, please test with removing xf86-video-intel (again, no one answered definitive yet if that is the common thread), reboot, and let the system use the modesetting (in-kernel) driver.
jotade70 When I said menu on the left, I meant the taskbar with the big "K" on its left. No trace of the taskbar nor response when pushing the "windows" key.
Logs? There should be something in the Xorg logs when a keyboard button is pushed. But, since you are not testing on a clean user, no indication if the Windows key is even configured to open the menu.