title: ScriptManager1.AsyncPostBackErrorMessage not showing error message
body: <p>I've always used the following two bits of code (which use to work) to catch Ajax <code>asyncPostBackError</code>s.</p>
  
  <pre><code>&lt;asp:ScriptManager ID="ScriptManager1" runat="server" OnAsyncPostBackError="ScriptManager1_AsyncPostBackError" /&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>protected void ScriptManager1_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e) 
  { 
  ScriptManager1.AsyncPostBackErrorMessage = e.Exception.Message; 
  }
  </code></pre>
  
  <p>But now even though the unhandled exception has been caught in this event handler function and the <code>AsyncPostBackErrorMessage</code> been set with the Exception message, I'm always getting the same error reporting in the page in a alert box no matter what the exception message was, saying:</p>
  
  <pre><code>Error: Sys.WebForms.PageRequestManagerParserErrorException: The message recieved from the server could not be parsed. Common causes for this error are when the response is modified by calls to the Respnse.Write() ....
  </code></pre>
  
  <p>The error is the same error you would get if you had an unhandled <code>asyncPostBack</code> exception and you didn't wire up the <code>Scriptmanger's</code> <code>asyncPostBackError</code> event handler method.</p>
  
  <p>No matter what I do I get the same error. What would be causing this? </p>