Simple method without an additional app, found by trial and error before reading this thread: In the "Notification panel" device settings, *untick* the already ticked "Brightness adjustment" checkbox, *then* (and this seems to be the crucial step that has not been mentioned so far in this thread) go back out of this "Notification panel" screen by pressing on the arrow symbol at the top left, thereby coming back to the device settings screen, then once more go into the settings of the "Notification panel" and tick the "Brightness adjustment" checkbox to reactivate it, then once more go back to the device settings screen by pressing on the arrow at the top left. Voila! This worked already several times on my Note 3 running Android 5.0.