Hello,

On Lubuntu 16.04 i had a few times the problem where the suspending the laptop (Latitude D600) not worked. It was working, turn off the screen, but not suspended compute,r i moved mouse cursor and there was login screen, computer not sleeping.

When i try again, i see:
GDBus.Error:org.freedesktop.login1.OperationInProg ress: There's already a shutdown or sleep operation in progress
When i kill process "systemd-sleep suspend" by command "pkill -f suspend" then computer immediately sleep, but when i return back from sleep, computer does not react, do not show desktop interface.

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I read someone advising disabling suspension on laptop lid close in Power manager (or setting HandleLidSwitch to "ignore" in /etc/systemd/logind.conf) or someone ran following:

xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/logind-handle-lid-switch -n -t bool -s true
but that is for xfce not lightdm