You can do that easily in the file display but not in the folder tree. In the folder tree you can't use double-click events. You can use the right-click context menu, a toolbar button or a hotkey, though.
On my machine the problem seems more simple: Opus always seems to show the no-drag (circle with a line through it) cursor when dragging with the right mouse button. I've sent a report to GPSoftware.
I see the same thing. Looks like either Opus or Explorer (which is handling the drop since it's on to the Desktop) is timing out or cancelling the transfer. I've reported this to GPSoftware as well.