Did you ever install a version of Opus earlier than 9.5 on that machine? Or did you upgrade that machine from Vista?
It's possible some incorrect registry settings are left over, if you did (although AFAIK they should get cleaned up automatically). Try the steps in How to manually remove Explorer Replacement settings and then re-enable Explorer Replacement in Preferences to see if it helps.
Also, make sure you are not running dopus.exe elevated as Administrator (Opus's built-in UAC support should be used instead) and also do not have any of the other Windows compatibility settings turned on for dopus.exe which may cause it to replace Explorer using the wrong methods.