T450 - Brightness control hotkeys not working after system wake

[Czechnology]

I have a repeating problem that sometimes after I wake my T450 from sleep, I am unable to change the brightness with the hotkeys ((Fn)F5,F6), while otherwise normally it's working. Pressing the keys has no effect on the brightness, even though sometimes - strangely - one of the keys displays the brightness meter without changing anything.
As a solution, I always have to open up Windows Power Options and pull the lever there to adjust the screen brightness. This is really annoying and sometimes outright embarrassing when you open the laptop to show someone something.

I've tried changing some drivers for the hotkeys but to no success. Anyone experienced this and managed to fix it?
(maybe it's a common problem but I wasn't able to find a solution yet)

Running Win7pro.

Thanks!