Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed. Common causes for this error are when the response is modified by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.


Have you ever got this kind of error when you are doing an Async postback using AJAX.NET (starting from Beta 2 if I’m not mistaken and going all the way to the current RC1 – at least at the time of writing this post) ?

It seems the newer versions of AJAX.NET really hates having “Response.Write” being called while doing an Async postback. It changes the response a bit causing it a hard time extracting the changed values and updating the changes using JavaScript on the client side.

If you get this error go through your page and all the controls in it (including controls within controls) and make sure you don’t have a “Response.Write” in the markup page.

Having “Response.Write” in the code behind in an event that is fired on an Async postback like Page_Load, Page_Render or the handler that handles that exact event can cause the same problem.

If you must use “Response.Write” you can get a similar solution by using a Label control and update it instead of using “Response.Write“.

If you use “Response.Write” in markup pages (ASPX or ASCX) replace them with either the “=” syntax (<% = MyProperty %>) or use the same solution mentioned above, adding a Label control and updating it in the code behind.