Check "webRequestModules" section registration in machine.config, and add if it does not exist.
<sectionGroup name="system.net" type="System.Net.Configuration.NetSectionGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
<section name="authenticationModules" type="System.Net.Configuration.AuthenticationModulesSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
<section name="connectionManagement" type="System.Net.Configuration.ConnectionManagementSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
<section name="defaultProxy" type="System.Net.Configuration.DefaultProxySection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
<sectionGroup name="mailSettings" type="System.Net.Configuration.MailSettingsSectionGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
<section name="smtp" type="System.Net.Configuration.SmtpSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
</sectionGroup>
<section name="requestCaching" type="System.Net.Configuration.RequestCachingSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
<section name="settings" type="System.Net.Configuration.SettingsSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
<section name="webRequestModules" type="System.Net.Configuration.WebRequestModulesSection, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
</sectionGroup>