I was able to start xfpm with the debug and no-daemon option in a console and got the logged messages (attached).
I'm afraid the log is not very helpful (AFAIK). I let the computer be idle and on battery for the specified time of 20 seconds and the brightness didn't change. Andres
$ xfce4-power-manager --no-daemon --debug TRACE[xfpm-main.c:203] xfpm_start(): Starting the power manager TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for general-notification TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for lock-screen-suspend-hibernate TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for power-save-on-battery TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for enable-cpu-freq-control TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for show-tray-icon TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for hibernate-button-action TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-enabled TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-on-ac-sleep TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-on-ac-off TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-on-battery-sleep TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-on-battery-off TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for dpms-sleep-mode TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for inactivity-on-ac TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for inactivity-sleep-mode TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for spin-down-on-ac TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for spin-down-on-battery TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for spin-down-on-ac-timeout TRACE[xfpm-xfconf.c:156] xfpm_xfconf_load(): Using default configuration for spin-down-on-battery-timeout TRACE[xfpm-power.c:1055] xfpm_power_get_power_devices(): Power device detected at : /org/freedesktop/UPower/devices/line_power_AC TRACE[xfpm-power.c:1002] xfpm_power_add_device(): device added: ((XfpmDeviceType) XFPM_DEVICE_TYPE_LINE_POWER) TRACE[xfpm-power.c:1055] xfpm_power_get_power_devices(): Power device detected at : /org/freedesktop/UPower/devices/battery_BAT1 TRACE[xfpm-power.c:1002] xfpm_power_add_device(): device added: ((XfpmDeviceType) XFPM_DEVICE_TYPE_BATTERY) TRACE[xfpm-power.c:1013] xfpm_power_add_device(): Battery device detected at : /org/freedesktop/UPower/devices/battery_BAT1: ((XfpmDeviceType) XFPM_DEVICE_TYPE_BATTERY) TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 124 : ((XfpmButtonKey) BUTTON_POWER_OFF) (xfce4-power-manager:4766): xfce4-power-manager-WARNING **: could not map keysym 1008ffa8 to keycode TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 213 : ((XfpmButtonKey) BUTTON_HIBERNATE) TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 150 : ((XfpmButtonKey) BUTTON_SLEEP) TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 233 : ((XfpmButtonKey) BUTTON_MON_BRIGHTNESS_UP) TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 232 : ((XfpmButtonKey) BUTTON_MON_BRIGHTNESS_DOWN) TRACE[xfpm-button.c:179] xfpm_button_xevent_key(): Grabbed key 244 : ((XfpmButtonKey) BUTTON_BATTERY) TRACE[xfpm-battery.c:150] xfpm_battery_refresh_visible(): visible=TRUE: ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-polkit.c:368] xfpm_polkit_init_data(): Using unix session polkit subject TRACE[xfpm-polkit.c:455] xfpm_polkit_check_auth_intern(): Action=org.freedesktop.upower.suspend is authorized=TRUE TRACE[xfpm-polkit.c:455] xfpm_polkit_check_auth_intern(): Action=org.freedesktop.upower.hibernate is authorized=TRUE TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-polkit.c:455] xfpm_polkit_check_auth_intern(): Action=org.freedesktop.udisks.drive-set-spindown is authorized=TRUE TRACE[xfpm-disks.c:170] xfpm_disks_get_is_auth_to_spin(): Is auth to spin down disks : 1 TRACE[xfpm-disks.c:147] xfpm_disks_set_spin_timeouts(): On Battery=0 spin_down_enabled=0 timeout=800 TRACE[xfpm-backlight.c:324] xfpm_backlight_brightness_on_ac_settings_changed(): Alarm on ac timeout changed 120 TRACE[xfpm-backlight.c:345] xfpm_backlight_brightness_on_battery_settings_changed(): Alarm on battery timeout changed 20 TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-polkit.c:238] xfpm_polkit_free_data(): Destroying Polkit data TRACE[xfpm-manager.c:363] xfpm_manager_alarm_timeout_cb(): Alarm inactivity timeout id 2 TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-disks.c:147] xfpm_disks_set_spin_timeouts(): On Battery=0 spin_down_enabled=0 timeout=800 TRACE[xfpm-dpms.c:74] xfpm_dpms_set_timeouts(): Settings dpms: standby=300 suspend=0 off=600 TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-manager.c:363] xfpm_manager_alarm_timeout_cb(): Alarm inactivity timeout id 1 TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon TRACE[xfpm-battery.c:286] xfpm_battery_refresh_icon(): Battery state 2 TRACE[xfpm-power.c:1148] xfpm_power_refresh_adaptor_visible(): Tray icon configuration: : ((XfpmShowIcon) SHOW_ICON_WHEN_BATTERY_PRESENT) TRACE[xfpm-power.c:1113] xfpm_power_hide_adapter_icon(): Hide adaptor icon
