Microsoft has admitted that there was a problem with the KB4535996 update that it released in late February. Microsoft admitted the issue in the update only a couple of days ago, several days after users first began complaining about problems after installing the update. Microsoft said that it is “aware of issues” with signtool.exe after installing

Source: Hot Hardware – Microsoft Admits Major Issues With Windows 10 KB4535996 Update, Users With Bugs Should Uninstall