Okey, you are probably here for answers so... long story cut short... fix it by going to your local IIS Application Pools -> Choose your Pool (ASP.NET v4.0) -> Advanced Settings -> Process Model -> Identity
I changed it to "Local System" to get it working. I am not advising you to do this cause that's a lot of power given to the Pool. The
This fix only works for access rights issues and does not work for all types of "HTTP Error 503. The service is unavailable." that might creep up in time.