The fix to my particular problem was at the bottom of this thread:
there is an second registry key, which will be checked:
"HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\Auto Update\RebootRequired"
This one comes from WindowsUpdate and should not exist (also rename it like no_RebootRequired)
then try again by klicking the button "Re-run" (no need to restart the setup.exe)