This happens to one of my folders with a lot of images inside
The individual image thumbnail is fine though
That is a bug in Windows 10 which also affects File Explorer. (Google finds many threads complaining about it on the web. Nothing from Microsoft about a fix so far.)
Clear Icon Cache (make sure it's the latest version for Windows 10) can sometimes fix it, but sometimes the problem comes back. Windows seems to randomly mess up the background / alpha channel when generating folder thumbnails and then caches the bad result so it keeps coming back until the cache is cleared or something causes the thumbnail to be re-generated; then the dice are rolled again.
Within Opus, you can also turn off Preferences / File Display Modes / Thumbnails / Generate folder thumbnails via the shell (provides 3D folder thumbs) to make Opus generate the thumbnails itself, although they won't look the same as the shell's thumbnails.
OK. Thanks