F.lux version 36.5 on OSX 10.11.1 is unreliable
- f.lux enables itself only after opening the 'f.lux preferences' window (not after running the f.lux app)
- most of the times, after enabling f.lux, the screen flashes for a few seconds, oscillating between the f.lux colours and the default OS colour profile, then f.lux remains disabled
- it seems f.lux is affected by changing the brightness levels (either automatically or manually)
- the app started misbehaving after one of the following two events: turning it off manually (for the first time) or connecting a secondary display through HDMI
You have to turn off automatic brightness in System Preferences > Displays.