If you exit Opus and then run it again via the Start Menu, do you get a default lister again or do you get whatever listers you had when you exited? If you get the right thing, do you then get that every time you reboot? (The problem may be that Opus is unable to save the window state at exit, or during shutdown.)
Is it definitely the default lister you are seeing? Sometimes other programs, or their installers, trigger folders to be launched at startup. Try turning off Explorer Replacement mode and rebooting to see if you then get some Explorer windows opening at startup. If so, the cause of those windows is external to Opus.