Your SharePoint 2013 farm network environment is highly secured. Network operations pushes through a new GPO one evening having new security configurations. Next day, you try connecting to your SharePoint 2013 site, and your browser displays HTTP Error 503 Service Unavailable. You try connecting to the farm's Central Administration, and experience the same error. You check the Application event log of the server hosting the web application, and you see the following:
Log Name: Application Source: Microsoft-SharePoint Products-SharePoint Foundation Date: [date/time] Event ID: 8305 Task Category: Claims Authentication Level: Error Keywords: User: [domain\your account] Computer: [machinename] Description: An exception occurred when trying to establish endpoint for context: Could not load file or assembly 'Microsoft.IdentityModel.Extensions, Version=2.0.0.0, Culture=neutral, PublicKeyToken=69c3241e6f0468ca' or one of its dependencies. Either a required impersonation level was not provided, or the provided impersonation level is invalid. (Exception from HRESULT: 0x80070542). Event Xml: . . .