That seems to confirm the issue is within, or at least triggered by, the Dropbox shell extension.
If you want to find out if it's the context menu or the icon overlay, you can re-enable both via ShellExView, then add to context menu extension's CLSID into Opus's context menu block list.
To get the CLSID, right-click the context menu extension in ShellExView and choose Properties. The CLSID is near the middle of the window that appears. You can then add it here in Opus (more details here):
After doing that and restarting Opus, if the problem comes back then the issue is probably with DropBox's icon overlay handler. If the problem remains gone, it could be with the DropBox context menu.