Brightness changes are shown by the indicator but have no effect on the display (Lubuntu, Intel card)

I have tried many of the proposed solutions that I could find on the web, but any of them worked for me. I detail here my problem and some of the things that I tried:

Relevant details:

TOSHIBA NB200

Lubuntu 16.04 and 18.04

Output of ls /sys/class/backlight/:

acpi_video0 intel_backlight

Problem:

When trying to adjust the brightness of the laptop, the brightness indicator appears in the upper right corner reflecting the changes, but no variation occurs on the actual display brightness.

What I have tried:

• Use the default keyboard keys to adjust the brightness.

• Create/edit the xorg.conf file (using as "Identifier" either card0 or the output of xrandr --verbose, see e.g. here and here in the answer provided by Lorem Ipsum)

• Use the xbacklight package and change the brightness with $ xbacklight -inc 10 and $ xbacklight -dec 10

• Manually edit the file /sys/class/backlight/intel_backlight/brightness

In the last two, if I later use the default keyboard keys to adjust the brightness, I can see how the changes were applied, but again no variation occurs on the actual display brightness.

I would really appreciate your help! Any suggestion?