Sorry, I can't help. I just wanted to add that I'm also having the same problem, on Windwoes 10. In my case, automatic, as well as manual color change (Shift-Alt-PgUp/PgDown) and also manual brightness control (Alt-PgUp/PgDown) no longer work. When I try to use the shortcuts, the notifications (e.g. for % brightness) show up the same as before, but nothing actually happens. This all worked for as long as I can remember, until a few months ago. I stopped using f.lux for a few weeks before that and when I tried to run it again, it was no longer working.
The only thing that still works is Vignette Mode. It may also be worth pointing out that the Night Light function built into Windows 10 also fails to work the same way (i.e. I can turn it on/off, change the setting, etc. but nothing happens).
I recently cleaned out the display drivers from safe mode -- although, come to thing of it, I've also performed an upgrade repair install of Windows (Insider Edition) since then. Only thing I haven't tried yet (since I only just thought of it) is to try from a different user account...and now I have. In fact, since the store app wanted me to log into an account, I downloaded the installer from the website instead (if that makes any difference) and installed it as regular software. The problem remains the same: hitting the shortcut keys brings up the usual notifications, but nothing happens.
I also don't recall there being anything in the event log -- but I'll check again...nothing.
Here's the driver info (copied via options):
NVIDIA 25.21.14.1735: NVIDIA GeForce GTX 1070 (20181211)
Intel Corporation 20.19.15.4549: Intel(R) HD Graphics 530 (20161110)
1 monitors
Generic PnP Monitor
Dimensions: 382 x 215
Year: 2016
Chroma:
R=0.636719 0.347656
G=0.333984 0.627930
B=0.154297 0.049805
W=0.313477 0.329102
sRGB gamut: 77%, AdobeRGB gamut: 54%