Today i had to install .NET Framework 3.5 on Windows 10 1803, i saw the same error on Windows 10 1807 as well in the past. The installer quit with the error code: 0x800F081F.
The solution to make Windows 10 able to install .NET Framework 3.5:
Go into the registry and goto the HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU folder. DoubleClick on UseWUServer (UseWindowsUpdateServer) and change the value to 0. You have to reboot the workstation afterwards.