I think there might be a misunderstanding here ...
I had backups, of course.
They worked perfectly fine with Windows 8.1.
I created a fresh Win 10 installation on my new machine and for the love of Christ, it wouldn't work. It generated the error message that I described.
That is probably the problem ... SOMEHOW, the MSF was hold together by stitches and good faith so far, but Win 10, in the end, broke it.
There are numerous factors that have to be taken into account here, especially the fact that, before I ever found this tutorial a while ago, I had some...
Well, I tried everything and it did not work.
However, I have to point out that I have been carrying the same MSF files for nearly 20 years now.
It started with an old proprietary e-mail client in 1995, imported the files into Outlook Express in 2000 and later into Windows Mail.
I have a problem.
I just installed Win 10 on my PC (a brand new Dell Workstation).
I have been using the instructions, on this page, to reinstate Win Mail for YEARS. It always worked. However, I never used Windows 10.
After installing Win 10, I downloaded all Win updates.