You could try flashing stock 6.0.1 firmware doing the following:
1) Find your firmware here (
http://www.sammobile.com/firmwares/database/SM-G920F/) and download the correct one. (Extract to desktop after)
2) Download Odin 3.10.7 (
http://odindownload.com/ ) and open.
3) Click 'AP' and select the firmware you downloaded (May take a few minutes for it to load)
4) Turn phone off, put into download mode (Hold Volume down, Home button and Power button) and continue
5) Connect phone to laptop (while in download mode) and when it says 'Added' in Odin, click start.
Only solution I could think of to fixing your problem, remember to back up all data if you're going to flash.
Also clear cache before flashing firmware. (Phone off, hold volume up and power) and select the option, then go ahead and do your thing