Managed to successfully upgrade a Web Application from .Net Version 3 to Version 4.5 and I get the following error:-

HTTP Error 500.22 - Internal Server Error

As ASP.NET aspiration abortion setting has been detected abortion pill costs that does not apply in Integrated managed pipeline mode

Module: configurationValidationModule

Notification: BeginRequest

Handler: ExtensionlessUrlHandler-Integrated-4.0

Error Code: 0x80070032

Solution was quite simple and found it here:-

StackOverFlow - An ASP.NET setting has been detected that does not apply in Integrated managed pipeline mode

Fix:-

In web.config add the following or make sure it exists:-

<configuration>
    <system.webServer>
        <validation validateIntegratedModeConfiguration="false"/>
    </system.webServer>
</configuration>