Microsoft Exchange Event id 3 WebHost failed to process a request

When you expirience the following issues in your Microsoft Exchange environment:

• You receive an error when you access Exchange Management Console or Exchange Management Shell.
• The Microsoft Exchange Mail Submission Service may not start.
• The Microsoft Exchange Transport Service may not start.
• Users cannot delete email messages from Microsoft Outlook Web Access and receive the following error message:  An unexpected error occurred and your request couldn’t be handled

and in Event Log you can find:

Event ID: 3 
Description:
WebHost failed to process a request.

Event ID: 1031
Description:
The service configuration file could not be loaded for the following reason: System.Configuration.ConfigurationErrorsException: Configuration system failed to initialize —> System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (C:\WINDOWS\Microsoft.NET\Framework64\v2.0.50727\Config\machine.config line 134) at System.Configuration.ConfigurationSchemaErrors.ThrowIfErrors(Boolean ignoreLocal)

Event ID: 1002
Description:
Unable to start the Microsoft Exchange Mail Submission service: System.Configuration.ConfigurationErrorsException: Configuration system failed to initialize —> System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (C:\WINDOWS\Microsoft.NET\Framework64\v2.0.50727\Config\machine.config

This mean that you have installed KB2756920, you can read about it more here: KB2756920 .NET Framework Update.

To fix those issues you need to install HotFix: KB2637518 Hotfix for Microsoft .NET Framework 3.5.1 on Windows 7 RTM and Windows Server 2008 R2 RTM