So, this is in fact a known Windows 10 bug.
Solution is quite easy:
Open a Windows PowerShell window by using the Run as administrator option.
Run the following commands:
Stop-Service -Force WAS
Remove-Item -Recurse -Force C:\inetpub\temp\appPools*
Sauce : https://stackoverflow.com/questions/13322937/http-error-503-the-service-is-unavailable
GLHF, I lost an hour on this...