I've been using Directory Opus for years now, ever since Magellan on the Amiga .
Anyway, a bug has suddenly appeared over the last few months that is intermittent. When you right click on a lister on the taskbar icon, and select Close Window, it doesn't actually close. Doing this a few more times (seems to be random), it eventually closes. Closing the Lister other ways; the X button in the corner, or via the File menu, it always closes.
I'm not doing anything special to Directory Opus, using the default settings for everything, no custom toolbars, or strange lister layouts, and I'm on the latest version (10.0.5.0.4497.x64) from the updater. I tried to fix it today, by reseting my preferences, unfortunately this made it a lot worse and the lister rarely closes through the right menu now. I'm at a lost of what it could be. My operating system is Windows 7 64-bit, and again. I don't have it customized or anything that isn't normal, and I'm not running autohotkey, windowblinds, or any program like them.
Something like that will happen if you run Opus elevated (rather than running it normally and using the Admin Mode to elevate individual Opus windows as needed). Are you doing that?
A quick way to check is to see if you get a UAC prompt when you create a folder under C:\Program Files. If you don't get a prompt, Opus is probably elevated (or UAC is turned off).
If you select Preferences / Launching Opus / Explorer Replacement / Don't replace Explorer, click OK and then reboot the machine, do you still see the problem?
Another thing to try: Un-pin and then re-pin the Opus taskbar icon, if it is pinned.
Do you have anything else installed that might replace or intercept Windows Explorer, or anything like alternative shell tools or things which hook the desktop/taskbar or similar to modify their behaviour?