I’ve got this error when i have ajax toolkit script manager on the master page while at the same time i’ve script manager on the content page. This is the primary cause of this error.

The work around this is to move the script manager which is located on the master page before the content place holder or if you have any user control which uses script manager as well, place before it. It should be placed on the top of anything which uses this control and the most important thing you need to change the script manager in the content page or user control to be script manager proxy