There is an excellent blog entry by Eilon Lipton. It contains of lot of tips how to void this error:
Read the comments too. There is a comment of somebody with the same problem: "I solved it changing server idle time of my app pool on IIS. It was only 5, so I incremented it and now works."
Asynchronous postbacks are exactly the same as regular postbacks except for one important thing: the rendering. Asynchronous postbacks go through the same life cycles events as regular pages (this is a question I get asked often).
Only at the render phase do things get different. We capture the rendering of only the UpdatePanels that we care about and send it down to the client using a special format. In addition, we send out some other pieces of information, such as the page title, hidden form values, the form action URL, and lists of scripts."
Most common reasons for that error:
- Calls to Response.Write():
- Response filters
- Server trace is enabled
- Calls to Server.Transfer()