I am having the same problem. Fences had worked nicely for years, then my HDD crashed and everything had to be reinstalled, including Windows 10 Pro. Now I get random black squares and sometimes a black background. Tech support could only say purge and reinstall, but the problem persists.
I have three displays on my system and the problem reliably presents when moving to other desktops.
I really would like to continue using Fences, but the black superpixels is a show stopper for me.