Just a quick note here to share the solution I've struggle a bit to found.
This HTTP Error 503 occurred just after the installation of Fall Creators Update for Windows 10.
Note this is not a solution for all HTTP Error 503.
The problem is directly caused by the Fall Creators Update. Microsoft is aware of that and the solution can be found here :
https://support.microsoft.com/en-us/help/4050891/error-http-503-and-was-event-5189-from-web-applications-on-windows-10
This HTTP Error 503 occurred just after the installation of Fall Creators Update for Windows 10.
Note this is not a solution for all HTTP Error 503.
The problem is directly caused by the Fall Creators Update. Microsoft is aware of that and the solution can be found here :
https://support.microsoft.com/en-us/help/4050891/error-http-503-and-was-event-5189-from-web-applications-on-windows-10