I have the same problem.
there is a workaround although.
add "acpi_osi=" at the and of the kernel boot parameters.
to permanently do this edit /etc/default/grub as root
and add "acpi_osi=" at the end of GRUB_CMDLINE_LINUX_DEFAULT
then run "update-grub" as root
after that brightness and all other hotkeys work
but the ambient light sensor is no longer detected.
But this is not a big problem since there is no standard
ambient light regulation of screen brightness anyway