Updated to V4.118, still the same problem.
At nighttime Alt PgUp/PgDn dims or brightens. It's 11:05 PM now and the range is 2% to 60%. But it doesn't dim as much as earlier this year, and does not become red tinted when dimmed, just remains neutral.
Changed the pulldown to "recommended colors". It was on "custom colors" before, for some reason. Alt PgUp, PgDn now change between 0 and 14% but the brightness of the display does not actually change.
In Features and Notifications, backwards alarm clock is selected, use display data for color accuracy, use GPU for better quality. Use Windows internal color table is not selected.
Driver info is:
NVIDIA 26.21.14.4112: NVIDIA GeForce GT 635 (20191024)
Generic PnP Monitor, 597 x 336mm, built 2013
Chromaticities:
R=0.679688 0.309570
G=0.209961 0.700195
B=0.147461 0.053711
W=0.313477 0.329102
sRGB gamut: 102%, AdobeRGB gamut: 99%
Generic PnP Monitor, 521 x 293mm, built 2017
Chromaticities:
R=0.650391 0.333008
G=0.316406 0.612305
B=0.152344 0.067383
W=0.313477 0.329102
sRGB gamut: 96%, AdobeRGB gamut: 66%
Winlimit=0, 0
Monitor 1 on NVIDIA GeForce GT 635: VCGT=0 gamma=1
Monitor 2 on NVIDIA GeForce GT 635: VCGT=1 gamma=1
Windows Build: 19041.329
Um, the cancel button on the f.lux Options panel is greyed out and the Done button doesn't dismiss it! :)
Same visual response on the HDMI and VGA monitors, one is not different than the other.
Windows night light is off. No difference whether the strength is 0, 50, or 100, f.lux is still prevented from working.
Turn night light on, f.lux still has no effect whether the strength is 0, 50, or 100.