Check that Opus isn't running with any compatibility settings turned on, as that will cause problems with Explorer Replacement. To check that, find dopus.exe under program files, right-click it and open the Properties dialog. On the Compatibility tab make sure none of the options are turned on. (In particular, "Run this program in compatibility mode for..." and "Run as Administrator" must both be turned off.) Do the same for dopusrt.exe and (if you're on 64-bit Windows) dopusx64.exe. If any compatibility flags were on, clear them, OK the Properties window, then reboot.
The problem may also be solved by manually removing the Explorer Replacement settings, though you shouldn't normally have to do that.