Windows 7 and Windows 2008 PCs that are running Symantec security software are having issues installing updates today. The issue is due to security signing. Windows 7, of course, is scheduled for the trash heap in February 2020. Symantec and Microsoft are working on a temporary fix.
Microsoft’s support doc: August 13, 2019—KB4512486 (Security-only update)
Microsoft and Symantec have identified an issue that occurs when a device is running any Symantec or Norton antivirus program and installs updates for Windows that are signed with SHA-2 certificates only. The Windows updates are blocked or deleted by the antivirus program during installation, which may then cause Windows to stop working or fail to start. Microsoft has temporarily placed a safeguard hold on devices with an affected version of Symantec Antivirus or Norton Antivirus installed to prevent them from receiving this type of Windows update until a solution is available. We recommend that you do not manually install affected updates until a solution is available.