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any policies for a pc to reboot after a user logs out?

no policies like that, its definitely crashing too as we can see it's an unexpected restart in event logs

Try running poolmon.exe maybe a driver is leaking memory we had this several years back and it turned out to be an ivanti driver, my colleague figured that one out using this tool and the fault originally went around the houses though in this case the screens went black after the vda gradually slowed down.