This issue sometimes occurs when you have a control registered as an
AsyncPostbackTrigger in multiple update panels.
If that's not the problem, try adding the following right after the script manager declaration, which I found in this post by manowar83, which copies and slightly modifies this post by larryw:
There are a few more solutions discussed here: http://forums.asp.net/t/1066976.aspx/9/10
~ Answered on 2011-10-24 20:25:53