Why do I get the error message ‘Server Application Unavailable The web application you are attempting to access on this web server is currently unavailable. Please hit the ‘Refresh’ button in your web browser to retry your request.’

By default, ASP.NET runs its worker process (Aspnet_wp.exe) with a weak account (the local machine account, which is named ASPNET) to provide a more secure environment. On a domain controller or on a backup domain controller, all user accounts are domain accounts and are not local machine accounts. Therefore, Aspnet_wp.exe fails to start because it cannot find a local account named ‘localmachinename\ASPNET’. To provide a valid user account on the domain controller, you must specify an explicit account in the section of the Machine.config file, or you must use the SYSTEM account.
For more details : FIX: ASP.NET Does Not Work with the Default ASPNET Account on a Domain Controller