I am using a late-2015 iMac, running macOS Sierra (although this problem was also present with El Capitan). Flux 39.984. The computer is work based, so it will log out periodically and go to a log in screen. If I come back to the computer (this typically happens overnight), the screen will be frozen in the typical flux colours. If I try to log in, the screen freezes and I see the spinning wheel - the only way to alleviate the problem is a hard reset.

So far, my only solution to prevent this is to set the time for log-ins to be as long as possible and a complete wipe/reinstall of the system did nothing to solve the issue. Frankly however, leaving my computer unlocked for long periods of time (especially if I am absent) is not ideal...

Any help is greatly appreciated!