I have a page with Ajax Tab controls, within one of the tabs is a webcontrol that as a Telerik RadGrid, with Edit forms pointing to another web control. That edit form also cont
Well, I am considering this error is coming in your script. If you are showing/hiding the update panel then it will give this error. The best solution is known to me which worked too is put your update panel into the content template of the other update panel which will be the parent of the update panel that you are showing/hiding.
Now if you will hide the update panel with id Panel2
it won't give any error.