FIX: OWA cannot be accessed after you upgrade Exchange Server 2013

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Symptoms

After you upgrade Microsoft Exchange Server 2013 on a server that is running Windows 2012 or Windows 2012 R2, Outlook Web App (OWA) cannot be accessed. Additionally, you receive the following error message:

Error: "Could not load file or assembly 'AntiXSSLibrary
Error: "Could not load file or assembly 'AntiXSSLibrary, Version=4.2.0.0, Culture=neutral, PublicKeyToken=d127efab8a9c114f' or one of its dependencies"

When you check the Application log, you find entries that resemble the following:

Log Name: Application
Source: ASP.NET 4.0.30319.0
Date: date time
Event ID: 1310
Task Category: Web Event
vel: Warningvel: Warning
Event code: 3008 
Event message: A configuration error has occurred.

When you try to browse any of the Exchange Back End Website virtual directories, you receive the following error message:

Page cannot be displayed

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Cause

This issue occurs if the upgrade post-setup process is not run for the Client Access role. In this situation, the "SharedWebConfig" file is missing on following path:

//%ExchangeInstallDir%Program Files\Microsoft\Exchange Server\V15\ClientAccess

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