[libvirt-users] VM display is blank when open (but Gnome Boxes thumbnails ok)

Hi, I also posted this question on the IRC #virt channel but have not received any reaction at the moment of posting this message to the mailing list. Since I last turned off my laptop (Dell XPS-15-9570 running Ubuntu 19.04) on Friday 10 May (I had to do it by long power press by the way), I am no longer able to get the display of my VMs: I am getting a black screen, even though I can see it running in the thumbnail of Gnome Boxes. You can see the described behaviour on the following video capture: https://youtu.be/Lv67g0foyzc. Troubleshooting Log from Gnome Boxes is available here: https://pastebin.com/9eWGYsJV. It was working all properly on Friday before I turn off my laptop. Any clue what could be wrong and how I could solve this issue (I digged the Internet for 2 hours without success)? Thanks in advance for your help! Kind regards, Michel