"Enter your passcode" without a passcode

Solution 1:

It's an issue of f.lux, but it seems there's no solution for the problem. And sadly I don't think this issue will be patched soon as Apple stopped f.lux team developing the app for iOS.

Solution 2:

It sounds like this is a bug.

Make a backup of your iPhone in iTunes. Reboot (shutdown and power up) the phone, then try setting a 4-digit passcode and locking the iPhone. Reboot the phone again and unlock the phone using the 4-digit passcode. Then, go into Settings and remove the passcode.

If this does not work (i.e. the phone prompts you for a passcode after this procedure) you are best off just backing up again and restoring. Otherwise, you should go and see Apple Support and get your phone replaced. Remember to backup first if you are! You may not have warranty anymore, but chances are if you tell them your problem, they'll just replace the phone for you.

By the way, just to answer your last question: I have not (yet, touch wood) but this sounds related. And I don't know if it can be cured, but give it a shot. Best of luck.