All you can do is try to download the full installer again
Today, SF 6.1.9.
4 has been released. So I downladed the streamfab_x64_6194.exe, then uninstalled my 6.1.9.2 copy and did a "full install" of 6.1.9.4. Surprisingly, this worked without any problems at the first attempt, while I failed with 9.1.9.3 serveral times.
Don't know, why, but who cares, so I just skipped V9.1.9.3....
...thanks fpr your help, everybody, I think this issue states "resolved" now. :D