One possible cause is that Opus has been run elevated, which would prevent it from recieiving the request to open the folder. How to check that, and other related details, can be found here: Why NOT to run Opus as Administrator under UAC
If that doesn't help, are you able to launch Opus in other ways, e.g. by creating an empty folder on the Windows desktop and then right-clicking it and choosing the option to open it in Opus?