Can you post your driver version and f.lux version?

Ah, it might seem like a laptop since it's a 640M GPU, but it's actually an All-In-One machine (ET2701INKI, to be exact). That's the machine currently running Windows 8.1. It runs a single built-in monitor. I guess realistically it's a laptop that just can't go anywhere and doesn't have a lid to close :) . The new machine is an Intel NUC5i5RYK mini-pc with two monitors—one hooked up by HDMI, the other hooked up by an HDMI-to-DisplayPort cable to the DisplayPort on the box. Both are Asus VS239H-P monitors. That's the machine with Windows 10. I also have a Yoga 2 Pro running Windows 10 I haven't tried F.lux on yet. I'll install it on that and see if things work fine there.

Do you have an antivirus running that might have f.lux as a false positive? And can you find an update to your display drivers?