ArcGIS Web Adaptor configuration page fails with the error 'HTTP Error 500.19 - Internal Server Error' 'The requested page cannot be accessed because the related configuration data for the page is invalid.'
See also technical article 40441.This error occurs when the application pool that the Web Adaptor is assigned to is configured to use .NET Framework version 4.0, but the Web Adaptor requires version 2.0.1) Open IIS Manager.2) Select the Application Pools node in the treeview.3) In the list of Application Pools, right click ArcGISWebAdaptorAppPool and choose Advanced Settings.4) Change the .NET Framework Version property to be v2.0.5) Click OK.6) Reload the Web Adaptor configuration page.