Comment 4 for bug 1384515

This is very likely not a bug in xfce4-power-manager, because when you log out/switch users /lock your session, you're going to a new VT, which means the default X11 settings are being used and xfce4-power-manager isn't running there (at least not at the greeter, it's only being started in the session).

Odd that you cannot change your brightness anymore within the session, I could also imagine that there are some driver issues. I presume this issue persists?