I had this problem as well on my machine at work.
Try doing the following it worked and allowed me to download
the .Net Framework 3.5 on my Windows 10 PC : Go into RegEdit and
set the HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU folder.
DoubleClick on UseWUServer(UseWindowsUpdateServer) and change the value to 0.
If the value is already 0 this fix may not work for you. RESTART afterwards.
Moderator's note: It is recommended to create System Restore point before modifying registry.