Marwan, your workaround did in fact fix the problem for me.
I'll add that when I tried your workaround (echo 0 >/proc/sys/kernel/nmi_watchdog), I REMOVED acpi_backlight=vendor (which I already had) at the same time. I rebooted and both problems were fixed: I could change the backlight using my keyboard AND changing the backlight no longer caused the computer to freeze. I haven't tried putting acpi_backlight=vendor back in my grub configuration to see if that breaks anything.
Marwan, your workaround did in fact fix the problem for me.
I'll add that when I tried your workaround (echo 0 >/proc/ sys/kernel/ nmi_watchdog) , I REMOVED acpi_backlight= vendor (which I already had) at the same time. I rebooted and both problems were fixed: I could change the backlight using my keyboard AND changing the backlight no longer caused the computer to freeze. I haven't tried putting acpi_backlight= vendor back in my grub configuration to see if that breaks anything.