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.

Answer 1

The action that causes this code to execute MUST be a postback event, and not an AJAX call.

This is due to the nature of the way AJAX requests are processed.

Answer 2

On your button click, redirect to another page that can stream any files that you might want to do this type of thing with. We use a document.aspx page in many of our sites and then pass a document id via querystring and stream the file from there.

In your example, you're basically trying to change the headers for a page that's already been displayed which isn't allowed

Answer 3

Keep your button outside the update panel. Then it works fine.