Finally I have found a very clear situation, that explains all symptoms and why anyone has a solution that may work and may not!
First: Dell LX501X has two video adapters, primary Intel, secondary Nvidia GT 420M.
For Nvidia, DO NOT use WindowsUpdate Driver, because it corrupts Registry; use last driver available at Nvidia Site.
BUT this has no relevance for brightness problem, that is related to the Intel Video Adapter only!
Do not search for other versions of video Driver! Someone referred to version 2239 or other previous to 2900.
The problem is not driver version: it is OK the WindowsUpdate 8.15.10.2900; the problem is not the "FeatureTestControl" value: F800 found in 1709, remains unchanged in 1803 and is ok, no need to change it!
The problem is Windows kernel, and I explain why.
We all agree that in 1709 all works fine.
In 1803 brightness does not work, BUT, if I REINSTALL the SAME video driver ALREADY INSTALLED (forcing installation, even if the same version: it is possible, selecting "have a disk"), after this action, brightness WORKS FINE.
BUT, it works ONLY until NEXT BOOT: after next boot, the problem is present again.
So, in conclusion, to have brigthness working, I have to REINSTALL video driver AFTER EACH Pc power on!
Everyone, I think, must agree that this behaviour is a Windows Kernel Problem (in the area of variuos modules hierarchy linkage)!
It is a BUG that MUST be corrected BY Microsoft!
BE AWARE that brigthness, when works, works ONLY by means of keyboard Keys: the various Windows sliders have no effect; the situation is VERY CRAZY: moving the sliders, brigthness does not change; using keyboard Keys, sliders move according to...!