If you are using AJAX enabled web site or project, you may come across this error message. I got this error message too and spent few hours to fix this problem. I tried finding all possible solutions on Google and finally concluded with following three steps.
- Web.Config: Make sure your web.config is correctly configured to handle AJAX enabled request. You might have created a project which was not AJAX enabled earlier and now you want to Ajaxfy. The best solution is creating a new project ASP.NET AJAX Enabled Web Site. It will create a Web.config file by default. Compare this Web.config file with your other project’s Web.config file which is casuing the error. Make sure that you are not missing any new section. You can simply copy and paste it at appropriate place. For detail information you can also visit at http://ajax.asp.net/docs/configuringASPNETAJAX.Aspx
- Using Master Page: If your ASP.NET web page is using a Master Page, make sure your Script Manager Control is placed in Master Page only. Content page should not be using Script Manager Control.
- Regular Web Page: If you are not using a Master Page then each ASP.NET web page must have its own Script Manager Control.