For some reason, Windows 10 is showing up as blocky, without any text, and with a solid white screen in Parallels 13 on OSX High Sierra. I happen to be running this on a Mac Pro 5,1 on a solid state drive (SSD) from OWC called Accelsior E2. I don't know if the SSD has anything to do with it, but it might. On this exact same computer I installed OSX High Sierra on an older traditional hard drive and didn't have this display problem. But the display problem doesn't happen at first. The initial Windows bootup screen looks fine. Then when Windows gets running, everything is blocky, with no text or images.
See a video of what happens here:
(open in 1440 resolution to see the whole thing clearly)
See a video of what happens here: