Microsoft.Office.Interop.Outlook --> 800700c1 is not a valid Win32 application

I "own" a C# WinForms Desktop app that has about 30-50 users using it per day (usually constantly during their shifts). One of the things it does is call Microsoft.Office.Interop.Word and/or Microsoft.Office.Interop.Outlook in order to open up a new Word or Outlook document and then populate that document with information. The user then can manually override before either saving the Word doc or sending the email.

For ONE user, it seems that he suddenly is no longer able to get the app to open up a new Outlook message. It's throwing an exception:

Retrieving the COM class factory for component with CLSID {0006F03A-0000-0000-C000-000000000046} failed due to the following error: 800700c1 is not a valid Win32 application. (Exception from HRESULT: 0x800700C1).

And it's referencing the line where we do: Outlook.Application outlook = new Outlook.Application();

I've been Googling for a few hours on this, trying to figure out "why just this one user?" and how to fix it. I had him run the Office 365's "Online Repair" feature. I had him try re-registering the Microsoft.Office.Interop.Outlook.dll file. No change. I found some articles out there that suggested if one's project is set to a build platform target of "Any CPU", that can cause the problem. So I created a special version just for this user where it was set to "x86" (as per the recommendation). No change. (Meanwhile, by the way, the app can hand off to Word no problem!)

I'm kind of at a loss as to what to do next to try to fix this.

Any thoughts?

So, my user decided to go make sure he was updated to the latest version of Windows 10. (Despite the fact that corporate has managed each device's Windows Updates for years now.)

But doing this solved the issue. It was indeed an OS installation issue.