I have always used the Terminal to install Electrum, by just following the instructions on the site. Yesterday hadn't mentioned it to avoid clutter I had also downloaded the Appimage. But when I clicked on it, nothing happened, nothing at all.
Have you tried to make it executable? Try to follow this guide:
https://discourse.appimage.org/t/how-to-run-an-appimage/80.
That 404 error is probably related to your network. You could also try to change the update server or use another DNS to connect to the update server.