Please provide me a solution how to fix this?
Edit "/etc/default/grub" as root.
Now, look for the line that looks like:
Append "acpi_backlight=vendor acpi_osi=Linux" to the end of that line, before the end quotes, so that the line now looks something like this:
Save and close the file, and run the following command:
Now, reboot, and test out the brightness keys. They should be working correctly now.
Have a look at this bug: https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/762670
It's for the XPS 17, but maybe it works on the 15" model too.