Could not load type 'System.Runtime.CompilerServicers.ExtensionAttribute from assembly 'mscorlib'

I keep getting this error approximately every other two executions of my web app. After that, the web app functions normally. However, I fear when we present our system if might fail. I have read articles about this error but I haven't resolved mine unfortunately. I am using Visual Studio 2010 with ASP.NET 4.0. I am also using AjaxControlToolkit.

The code highlights the ToolkitScriptManager.

<h4>RATE SERVICE PROVIDER</h4>
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"> <!-- this one
</asp:ToolkitScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">