Check that Windows is not configured to run Opus under any compatibility mode, which can cause problems with Explorer Replacement:
Also make sure Opus is not elevated under UAC:
Reinstalling Opus over the top of itself, and then rebooting, may also help if the problem is with registration/installation. Rebooting is important, don't skip it.
Manually removing the Explorer Replacement settings may also help, though probably only if the problem was due to Windows compatibility settings: