We used Ajax in one of our ISV application and when we deployed it in Production server, we started facing this issue, after doing a search in google, I found I need to do changes in my webconfig file to make ajax enabled, as we used vs 2010, so these entries were not there in our webconfig file. So we created one sample website in VS 2008 and all required configuration was there in webconfig. Then we copied these required configuration in our ISV application and deployed to production server, then we started getting another error

“There is a duplicate ‘system.web.extensions/scripting/scriptResourceHandler’ section defined”

After commenting this section it worked perfectly.

Enjoy!!