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.