If you are still having problems, use the creation tool to make a bootable USB drive (16 Gb minimum) with the latest 1809 version on it, then use Windows Explorer to view the file contents of that USB drive; right-click on the setup.exe file and choose "Run as administrator". If that also fails (with the same or a different error message), reboot and try it again by the self-initiating process; if that also fails, taking the image to some point even further back may be required.