This affects the Dropbox's folder and the descendants. Apologise for my English.
The Dropbox (or possibly another program's) icon/overlay shell extension is most likely failing.
You can use ShellExView to see which icon and icon-overlay extensions are installed and try blocking them to see which one is causing the problem. (You should fully exit Opus after each change in ShellExView.)
Before doing that, make sure the Dropbox software is up to date. Updating Dropbox has fixed problems people have had in the past.
Icon/overlay extensions for other cloud storage and drive-imaging products have been known to cause problems as well, so if you have more than one installed they are worth investigating as well.
Thanks! After using ShellExView and "enabled" the Dropbox's stuff, the icons are now shown.