Do you mean right-clicking on files & folders within the file display (right-hand pane), or do you mean the menu that appears when you right-click an empty space within the file display?
When you test with Windows Explorer, are you right-clicking on the same things as in Opus?
It sounds like the kind of problem that a context menu shell extension could be causing. The steps in this FAQ may help you narrow down which extension is going wrong, if that is the case.