Securitytoken.svc cannot be activated due to an exception

“SharePoint 2010 Server stops working after installing KB 2756920”
“Problems with SharePoint 2010 Server after installing Windows Updates, like – Errors in the navigation panes – Complete failure to open any site – SharePoint sites will return 503 server error.When you check the Server’s Event- and/or ULS logs, you may find some errors similar to these:

Sender Information: System.ServiceModel.ServiceHostingEnvironment+HostingManager/17653682
Exception: System.ServiceModel.ServiceActivationException: The service ‘/SecurityTokenServiceApplication/securitytoken.svc’ cannot be activated due to an exception during compilation. The exception message is: Method not found: ‘System.String System.ServiceModel.Activation.Iis7Helper.ExtendedProtectionDotlessSpnNotEnabledThrowHelper(System.Object)’.. —> System.MissingMethodException: Method not found: ‘System.String System.ServiceModel.Activation.Iis7Helper.ExtendedProtectionDotlessSpnNotEnabledThrowHelper(System.Object)’.”

Support for Microsoft Windows Server 2008 R2 RTM +SP1 started on Febr. 22nd 2011!  you need to be at least on SP1.
Check out the lifetime support matrix here:

For Details: