I had a similar problem recently where my stills were nothing but green and orange lines, all messed up. When I scrubbed them they would change to scenes from the clip, but when applied to a clip, the clip would look the same, green and orange lines. When I opened previous timelines, I could make stills just fine. And since then, stills have worked just fine, it was only that timeline and those clips, so it must have something to do with the clip itself having corrupt information, I'm guessing.
Try loading a clean version of one of your clips, don't do anything to it, and try grabbing a still and see what happens. This way you'll know if it's the clip, or if it's being caused by whatever grading you've done to it.
Dell XPS 8920
Intel i7 core 7700 build
GeForce GTX 1050ti
32 Gigs of RAM
3 7200 RPM SATA Drives
Windows 10 64-bit
DaVinci Resolve 15.2
Thanks for the response. I can give it a shot, but this is happening for all clips, and even across Resolve projects. As for the files, they are the .braw files from Black Magic, specifically made with Resolve 15 in mind!
Maybe a fresh install is needed, though we installed it clean only last month.