Since the latest Windows 8 update (12/03/2015) my Directory Opus is no longer the default explorer and cannot be reset to be the default explorer. My system has reverted back to Windows Explorer. Running Opus 18.104.22.168.3889.x64
You can try turning Explorer Replacement off and then back on again; failing that, unfortunately Opus 9 is not supported under Windows 8.
Turning Explorer Replacement off and then back on again did not fix the issue.
I downloaded Version 11 Trial and the folders continue to open up in Windows Explorer.
If Opus 11 is still on the system, please run FileTypeDiag and click save without typing an extension, to save a list of your folder-related registry settings. If you zip the result and post it here we can see if the problem is in the registry.
Please also make sure Opus is not running with UAC elevation, as that can break Explorer Replacement. (Opus has its own UAC support built-in, and the main Opus process should always be run normally, without elevation.)
Note that Opus 9 is very old and not supported for use on Windows 8. You need to upgrade to Opus 11.
I do recall this happening once before, earlier this year. The culprit was windows update kb3021674. I uninstalled it and DO went back to being the default explorer. I don't know which kb it is in the latest updates but I have restored my system to an image I made 4 days before the latest updates. DO is working as it should again. I have disabled automatically install updates.
I don't know if uninstalling/not installing important updates is the right solution. Mainly your sys becomes vulnerable and the more updates to come (or when updating to free Win 10) the less DO9 could work.