Do you see this in all folders, including under, say, My Documents, or only in some?
Opus won't show a menu for files that cannot be accessed (opened with read permission). (Explorer will show a menu, but almost nothing in the menu will work.)
If it happens everywhere, please try turning on Preferences / Miscellaneous / Windows Integration / Hide Windows items on file context menus (shift overrides) to see if that helps.
If that helps, the Finding the culprit section here has the steps needed to track down the shell extension causing the problem.