I also have this problem when trying to use more than one updatepanel.  I originally had 2 sets of GridViews/DetailsViews controls each set in a separate updatepanel.  This worked ok until I incorporated the CuteEditor. Then I got the error messages. Once I used only one updatepanel for both sets the problem went away.  It's not ideal, but it's what I had to do to get it to work.