Server Error in '/ArcGIS/rest' Application.
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Server Host cannot be null. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: ESRI.ArcGIS.ADF.Connection.ServerHostNullException: Server Host cannot be null.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace: 


[ServerHostNullException: Server Host cannot be null.]
   ESRI.ArcGIS.ADF.Connection.AGS.AGSServerConnection.Connect(Boolean throwExceptionOnUnauthorizedAccess) +364
   ESRI.ArcGIS.ADF.Connection.AGS.AGSServerConnection..ctor(String host, Identity identity, Boolean connect) +41
   ESRI.ArcGIS.REST.RestAppConfig.CheckSecuritySetting(HttpContext ctx) +81
   ESRI.ArcGIS.REST.RestAppConfig..ctor(HttpContext context) +58
   ESRI.ArcGIS.REST.RestUtils.GetRestAppConfig(HttpContext context) +76
   ESRI.ArcGIS.REST.RestModule.System.Web.IHttpModule.Init(HttpApplication app) +28
   System.Web.HttpApplication.InitModulesCommon() +66
   System.Web.HttpApplication.InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) +814
   System.Web.HttpApplicationFactory.GetNormalApplicationInstance(HttpContext context) +243
   System.Web.HttpApplicationFactory.GetApplicationInstance(HttpContext context) +106
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +340
 

INCORRECT rest.config file - NO SERVER:

<GISServer>
    <ServerName></ServerName>
    <SoapUrl></SoapUrl>
    <SoapSslUrl></SoapSslUrl>
    <Identity>
      <EncodedString></EncodedString>
    </Identity>    
  </GISServer>

CORRECT - with server:

  <GISServer>
    <ServerName>GISINTRA2</ServerName>
    <SoapUrl>http://GISINTRA2/ArcGIS/services</SoapUrl>
    <SoapSslUrl>https://GISINTRA2:443/ArcGIS/services</SoapSslUrl>
    <Identity>
      <EncodedString>f8LkonWzJ0sHHuO9t2vYMWIMrXCx2GkJ5vE2vEZQ2DQ=</EncodedString>
    </Identity>
  </GISServer>

<GISServer>
    <ServerName>{hostname}</ServerName>
    <SoapUrl>http://{hostname}/arcgis/services</SoapUrl>
    <SoapSslUrl>https://{hostname}:443/arcgis/services</SoapSslUrl>
    <Identity>
      <EncodedString>{yourstring}</EncodedString>
    </Identity>
  </GISServer>