Assembly Problem

you can installed System.Web.Extensions.dll  into the Global Assembly Cache (GAC) otherwise they won't have enough privilege to run some parts of themselves correctly.

The element creates a configuration section and subsections for the
SystemWebExtensionsSectionGroup class. You set the properties for these sections
in the element.

The element provides elements that can be uncommented
to configure how Web services are called from Microsoft ASP.NET AJAX applications.
The element is used to specify custom type converters and to
override default settings for JSON serialization. The
element specifies custom type converters. The element is used to
enable or disable the authentication service. The element is used to
enable or disable the profile service and to specify which properties are exposed by the
service. The element is used to enable or disable caching and
compression of resources used with scripts.

The element, element,
and element can be defined only in the Machine.config file or
in the Web.config file in the application root directory. The
element can be defined in the Machine.config file, in the Web.config file
in the application root directory, or in a Web.config file in a subfolder of the Web site.

The following example shows the element for ASP.NET AJAX.
Add this section and its subsections to the existing Web.config file as a child of
the element.