The type initializer for 'AjaxControlToolkit.ToolkitScriptManagerConfig' threw an exception

Doing research on internet, I found ajax control toolkit requires full trust to function. The default trust on server is medium, you need to change that to full. Add <trust level="Full" /> under system.web in your web.config.

The default ASP.NET trust level is set to Medium. 

In general, most applications will run fine under Medium Trust. If your application requires Full Trust, you can override the trust level at the application level by adding the following to your web.config file: 

<configuration>

<system.web>

<trust level="Full" />

</system.web>

</configuration>