That’s the MS Office viewer crashing when you select the file, by the sound of it. It’s used by Opus (if the preview pane is open and you select an Office document) but not actually part of Opus.
Can you view Office documents in File Explorer’s preview pane? If not, reinstalling Office usually fixes problems with its viewers and other components.
The crash is still happening when the Word viewer reloads the file. Have you tested the same thing, with the preview pane open, in File Explorer?
There should also be some crash logs which you can send us for analysis. Click “Opus FAQs” at the top of the page and it’s in the short pop-up list.
Don't worry, I found out there is an issue on our side, due to a change in the most recent beta. We'll get that fixed in the next beta and how it talks to the Word viewer. If it's causing problems, please drop back to 12.28 until 12.28.3 or 12.29 are out.