I am experiencing a problem on three installations that DOpus is no longer the Windows Explorer replacement as before. In other words Windows Explorer opens rather than DOpus - for example a shortcut on the Desktop. All the machines are running Windows 10. DOpus = version 11.18. I have tried replacing all settings from a backup, uninstalling and reinstalling DOpus but without any success.
Please try setting Preferences / Launching Opus / Explorer Replacement: Don't replace Explorer, then Apply. Wait 10 seconds, then set it to Replalce Explorer for all folders, Apply, and wait 10 seconds.
Does that repair things? (It can also sometimes take a few more seconds, or a few attempts, before the shell notices the registry changes, so it's worth closing any open Explorer windows each time and trying again a couple of times, but if it's still not working after 3 or so attempts then trying more isn't worth it.)
Assuming UAC is enabled, you should also make sure dopus.exe is running under your normal account and not elevated. See here for details. (Admin Mode and general UAC usage within Opus are both OK, but launching the whole program elevated will mean that non-elevated programs cannot ask it to open folders.)
Failing both of those things, I would look for commonalities between the machines, e.g. the same anti-virus or the same utilities which might be interfering with what happens in some way.
I am experiencing the same thing with my current win7 laptop; it started about 4 days ago. I reviewed Leo's post, but I don't observe any UAC issues. I do notice though that desktop icons for my web links have all changed, and this happened at the same time. I use Mozilla Firefox and experienced remappings to the Developer version of Firefox, which I use sometimes but not normally. ss
Assuming Opus 11.18, does it say "ADMINISTRATOR" in the Opus window titlebars?
And if you right-click a folder on the desktop, is the "Open in Directory Opus" menu item present? Does selecting it work, or open Explorer (or something else)?
I am definitely NOT running Opus 11.18 in Admin mode. I confirmed this by selecting the Admin icon, which does accomplish this and show the "Administrator" prefixed to the path string. (This is a great feature, by the way).
Right-clicking a folder shortcut on the desktop does not have the "Open in Directory Opus" menu item present. However, selecting the sub-menu option "Open Folder Location" does boot Opus, but just double-clicking the shortcut doesn't... At least the right click and sub-menu is a quick workaround. 
Via Preferences / Launching Opus, turning off Explorer Replacement, clicking OK, then turning it back on may fix things in your case.
It sounds like some of the registry entries have been removed by something.