No, I'm not using any cloud syncing tools.
Further information: In DO, I can switch between various views (large/small icons, thumbnails, tiles) and the folder icons show up just fine. It is only the list, details, power views--which all use the same icon size--where icons are missing. As stated above, there are no missing icons in Windows Explorer.
It's indeed strange only some icons are missing... I cannot find any common denominator which would identify why only some icons, all of which are the same size, are missing.
Note that It is only the folder icon that exhibits this problem.
I just discovered this: If I move a group of folders, some with missing icons, some not missing folder icons, to another drive, then move them back again, all icons are "restored" and show as they should!
If all of the icons were not actually the same image, I'd suggest this problem is an issue concerning display resolution (1680 x 1050).