It doesn't looks like the Word preview handlers are installed, which is strange if they are working in Explorer on the same machine.
Try running the Office installer and using the Repair option.
If that doesn't work, I would try a trial version of Opus 12. It's possible the version of Office preview handlers you have depends on some changes made in the many years since Opus 10 which I've forgotten. (Be sure to make a config baclup first so you can go back to Opus 10 if you decide to do that.)
How do I run the installer for "Microsoft Office 365"?
It is very strange that preview works in Opus 10 on my old computer, which also runs Microsoft Office 365.
The same way you installed it the first time. It might also have an entry under Settings / Apps (where you go to uninstall things) with a Modify option there to repair the install.
I have installed Opus 12, and the viewer pane worked.
After that, I installed Opus 10 (again), and the viewer pane does not work.
Apparently it is not possible to install a functional Opus 10 today, but all existing Opus 10 installations work as they should?
What should I do?
I doubt it has anything to do with the installer. There is probably something different between your two machines / Office installers which Opus 12 handles but Opus 10 didn't. Maybe something we improved in the last five years, but I can only really guess from here.
Note that the versions of Windows and Office you are using Opus 10 with did not exist when Opus 10 was written. The initial Opus 10 release was in 2011, 7.5 years ago, and was last updated in 2014, a year before Windows 10 came out. Office 365 is currently the 2016 version of Office. There's no way we could have anticipated everything that those versions might need before they existed.
No worries. I have bought an upgrade.
I just have one question left: What do you think your logo shows? A calculator in front of a tilted evening sun? 
I have been wondering every day for the last 15 years, so I deserve an answer now.
In my new Opus 12, the "Operations" tools bar continues to pop-up, although I have un-selected it.
How can I force Opus to remember that I have removed the tick-mark for "Operations"?
Settings > Toolbars > Set as Default Toolbar Set.
Please use a new thread for new questions unrelated to "How do I see Word files in the Viewer Pane?" 