I'm getting the following error message in the Output window of VS
A first chance exception of type 'System.ArgumentException' occurred in Coolite.Ext.Web.DLL
On my page, I get a MessageBox titled 'Request Failure' with a big red X icon and it says 'Status Code: 200' and 'Status Text: OK' (screen capture attached below)

My ScriptManager on the page is set to debug
<ext:ScriptManager runat="server" ScriptMode="Debug" />
In my web.config, I have customErrors turned off and I have compilation debug set to true
<customErrors mode="Off">
      <error statusCode="404" redirect="/404.aspx" />
    <compilation debug="true">
I have no idea what is throwing this error.

Is there a way to throw the actual error message instead of showing the MessageBox without any details of the error?