I got this error with a .Net MVC application that uses Kentico as a satellite CMS. The issue was that it needed a proper license for the URL my app was on. It basically stopped the app and caused IIS to return nothing but this 503 - service not available message. I ended up finding the error details in Kentico's Event Log (as a warning). You can create a new license or find license details in Kentico's client portal - https://client.kentico.com/.