Comment 7 for bug 1605189

I have the same problem using XFCE on Ubuntu 17.10. But Markus's solution does fix the problem.

My setup is as follows:

In XFCE Power Manager under the security tab set:
Automatically lock the session to "Never"

Uncheck "Lock the screen when the system is going for sleep".

Under the display tab I still blank the screen after 15 minutes. Put to sleep and Switch off times appear to be disabled (greyed out).

Under the system tab I still have system sleep mode going to "Suspend" after an hour.
Interestingly the screen still locks when it suspends.

So everything still works fine and as I want it to. I get a full screen white on black message when I activate the machine again telling me the screen is locked and to wait a few seconds for a dialog box to appear. Haven't seen that before but fine nonetheless.