Usually this means the still is missing. The thumbnail is saved in the project for reference but you may have changed or deleted the still folder as Resolve can't locate it where it was stored. If you grab a new still and that displays correctly its leaning toward the black ones being missing.
I actually had this happen in an entire project recently. Conformed from fcp7 XML, no stills at all would play, just black. Same media in a different session would still play just fine, but not in that particular session. I was running 8.2.1 beta 2 & 3 (I updated to see if it was a bug).
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Could you check if stereoscopic left or right eye are enabled in these sessions where the issue persists (assuming its a non-stereo project)? If that is the case, toggling these off should correct the issue.