For a year I have been using a heavy hammer to view system logs (both GNOME Logs and KSystemLog):
$ xhost +SI:localuser:root
I just saw https://tracker.pureos.net/w/troubleshooting/gnome_logs_can_t_see_any_logs/ , which is a much nicer and more specific solution. I expected immediate effect, but that didn't happen, so I logged out and re-logged in for the usermod to take effect. Maybe there is a time delay, but if logout/login is needed, please add a note to the end of the instructions.