>From one of Sergey's messages here I found also

Not sure this would apply here - opening them readonly, what was a merge was merged long ago, I'm just going through files and printing them to pdf.

Here, got a couple of screenshots. The "Show help" says

This error may appear if you attempt to open a file marked as damaged or disabled by a Microsoft Office system program. Each time an attempt is made to open the file ? even if the file was recovered or repaired ? will cause this error to appear and other currently open files or programs to suspend.

The funny thing is that I haven't even touched OneNote (and I don't even know what's that supposed to do), ever, not that I remember, and my simplest version of Office doesn't even have ?ms project? or ?ms sharepoint designer? (but I'm funny-punctuation-compliant, started ?using? question ?marks? as delimiters).

And it's not that that I didn't wait for the "Microsoft Office system program" to tick away its ten minutes. I did, and I then erased the bad document, then let my code process the other files, which took more than 40 minutes (first run, and print dialog not in the background and the few funny documents sitting there waiting for another click as per #2). Then I came back, unzipped this bad document afresh, but no - Word is a zlopamtilo (remembers the evil longer than the good) and popped the dialog again.

The other dialog is on the third image - which I was eventually able to catch; seems to be the meaning of the dialog with the funny name is to suppress that particular dialog. They probably have one way to open a file without this or that dialog, but no catch-all general case...