It seems I understand the issue now. Since it seemed odd that two users had the same typo (I assumed it was a typo), I researched a bit, and it seem the illegal command has been introduced by the dreaded Migration Tool:
Other users posted on the internet part of their FSX.CFG that looked like that:
SimObjectPathsRemarks=These entries were automatically added from SimObjects.cfg by Migration Tool. Set
Now, this is entirely, and utterly wrong!
If they intended to comment a line, the proper way would have been to prefix it with a comment. Our installer, of course, properly recognize the two most common forms of comments on an .INI file ( the FSX.CFG it's just a standard .INI file with a .CFG extension ), which are the semicolon ; and the double slashes //
Putting a comment AS IF it was a keyword using the same official keyword as the sim *INSIDE* their newly made up keyword "SimObjectPathsRemarks" , it's just wrong, because it will easily confuse any parsing routine searching for proper keyword.
Of course, we'll adapt the installer, to defend ourselves better again this kind of corruption of the default CFG files, in this case clearly made intentionally by another product.
