Perfect, thanks! The attachment in comment #19 confirms it's a kernel
bug relating to the qxl driver:
Jun 09 16:22:59 bionic-vm kernel: INFO: task Xorg:1172 blocked for more than
120 seconds.
Jun 09 16:22:59 bionic-vm kernel: Not tainted 5.4.0-33-generic #37-Ubuntu
Jun 09 16:22:59
attaching both files
journal.txt got via ssh during the freeze
prevjournal.txt after force system down and reboot
** Attachment added: "journal.txt"
https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1881813/+attachment/5382185/+files/journal.txt
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** Attachment added: "prevjournal.txt"
https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1881813/+attachment/5382186/+files/prevjournal.txt
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