@KatharinaS2 Hi Katharina - I think you are referring to "Darkroom mode". Two things:
You can always exit this mode using the "disable f.lux" hotkey which is alt-End (Or a reboot if that is hard to get to)
Can you post f.lux options > driver info so we can see what's going on with the tablet?
@herf Hey Herf! Sorry for the late reply, I'll try downloading the display links drivers and changing the registry keys as suggested by you in that post. Thanks!