You have 2 backlight interfaces. Lets try to find out which one is the valid one then remove the other one.
Look inside /sys/class/backlight/intel_backlight and you should see a number of files including brightness, actual_brightness and max_brightness.
max_brightness contains the maximum brightness value that can be processed by the interface.
actual_brightness is the current brightness level
...where <value> is a valid value between 0 and max_brightness. Does this change the screen brightness?
echo <value> | sudo tee /sys/class/backlight/intel_backlight/brightness
If not, try the same with the acpi_video0 interface.
If still nothing, boot with the acpi_backlight=vendor kernel parameter and try both again.