Comment # 35 on bug 1180591 from Marcel Kuehlhorn

(In reply to Miroslav Bene������ from comment #34)
> It has been a while. There is a much newer kernel version in TW now. Does
> the issue still persist with that? It could be the case, since it looks like
> a problem with BIOS or ACPI.

In fact I have just recently also started seeing this issue on Tumbleweed

It was still working about a week ago for me and now neither the
xfce4-power-manager nor xbacklight are able to change screen brightness

> # xbacklight -set 100
> RANDR Query Version returned error -1

Using the method of writing to /sys with echo does work for me:
> echo 378 > /sys/devices/pci0000\:00/0000\:00\:02.0/drm/card1/card1-eDP-1/intel_backlight/brightness