Comment 13 for bug 1638136

In my case this turned out to be a known bug with the "modesetting" graphics driver, a fix for which would be available in linux 4.11 earliest:
https://bugs.freedesktop.org/show_bug.cgi?id=96572
Perhaps xbacklight works out-of-the-box on Ubuntu 17.10 :)

Googling around suggests Ubuntu 16.04 switched to modesetting driver recently (but I could not find definitive info on this).

Switching back to the intel driver fixed the issue for me: I edited the file
/usr/share/X11/xorg.conf.d/10-intel-modesetting.conf
to load the "intel" driver instead of "modesetting" driver.