Unfortunately, it’s not possible to reopen that case; I cannot use the Qubes issue tracker to ask for support.
I have a small update: if I suddenly disconnect the HDMI port of the second monitor, the dom0 GUI is recovered on the primary monitor.
After the GUI recovery, I can reconnect the second monitor, and I can work while waiting for the next issue 
In the journal log, I can see the following messages:
gen 03 18:18:40 dom0 kernel: i915 0000:00:02.0: [drm] ERROR [CRTC:131:pipe B] flip_done timed out
gen 03 18:18:55 dom0 kernel: i915 0000:00:02.0: [drm] ERROR flip_done timed out
gen 03 18:18:55 dom0 kernel: i915 0000:00:02.0: [drm] ERROR [CRTC:131:pipe B] commit wait timed out
gen 03 18:19:05 dom0 kernel: i915 0000:00:02.0: [drm] ERROR flip_done timed out
gen 03 18:19:05 dom0 kernel: i915 0000:00:02.0: [drm] ERROR [PLANE:82:plane 1B] commit wait timed out
gen 03 18:19:07 dom0 systemd[1]: Started getty@tty5.service - Getty on tty5.
In this log, I can see more error messages, but in a nutshell, it seems not helping…
Attached, you can find the complete journal log.
journalctl_03_1.log (682.7 KB)
Reading the “GUI randomly freezes” issue, I have seen:
Looks like a GPU driver problem, may be related to IOMMU. Try adding iommu=no-igfx to the hypervisor command line (options= in /boot/efi/EFI/qubes/xen.cfg).
Moreover, I have found this case in the forum:
So now, I have added the iommu option to /etc/default/grub:
GRUB_CMDLINE_XEN_DEFAULT=“$GRUB_CMDLINE_XEN_DEFAULT iommu=no-igfx”
compile and reboot…