Atom changed the location of their installer from https://github.com/atom/atom/releases/download/v1.25.5/AtomSetup-x64.exe (https://github.com/atom/atom/releases/download/v1.25.5/AtomSetup-x64.exe) to https://atom.io/download/windows_x64 (https://atom.io/download/windows_x64)
Thanks for the heads-up ! it's fixed now.