Excellent!
I'm not sure it was directly related to Opus. When launching Control Panel with Explorer Replacement turned on, the request to open a folder gets intercepted and routed to Opus, and Opus then says, "actually, I don't want to open this myself so I'll ask Explorer to open it after all." Opus then asks Explorer to open Control Panel in a similar way to what it does when you use the toolbar/menu item which does the same thing.
Something was broken in the registry settings for folders which meant that when Opus asked the OS to open the Control Panel folder it was failing. It would probably also have failed for any other program that asked for Control Panel to open via the same method.
Explorer itself takes shortcuts and does not depend on the same registry settings that everything else does when launching folders, so it was still able to open the Control Panel folder (provided it handled the request itself, without it being diverted to Opus).