Comment 2 for bug 908637

I just found, that gnome-settings-daemon was running on my system and that killing it got me rid of that issue. However I still think xfce4-power-manager-settings should have told me that there was an issue with the configuration. Also, when starting gnome-settings-daemon on the command line and waiting 10 seconds, the following pops up as soon as the display backlight gets dimmed:

(gnome-settings-daemon:16612): power-plugin-WARNING **: IsInhibited failed: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files

I also found the backlight dimming options in gconf-editor in /apps/gnome-power-manager/backlight and deactivated all entries, which didn't help.