Comment 8 for bug 1319598

As suggested by the output in comment #5, the issue seem to be wrong permissions in policykit:

TRACE[xfpm-polkit.c:455] xfpm_polkit_check_auth_intern(): Action=org.freedesktop.upower.suspend is authorized=TRUE

Not sure what exactly sets those policies, the power manager only reacts to it.
Same goes for light-locker, neither of those two should touch the policies, only react to them.

You could check the permissions that are set in /usr/share/polkit-1/actions/org.freedesktop.login1.policy (you need sudo-rights to do that) and see whether that helps with the problem.