I discovered that if you installed a program from Iobit (like Driver Booster, for example) it creates a service called "LiveUpdate" with full name, as I remember, "LiveUpdateSvc".
Even if you remove the program the service will remain and this cause the KB4103721 update to fail to boot Windows after the update.
Uninstall all Iobit programs and remove the service from the administrator command prompt typing "sc delete LiveUpdateSvc" (check it is the correct service name) and then you will be updating Windows without any problem.
I experienced this issue on two computers and this has been the solution.