does simultaneous ajax updates are possible on this page. Looks like, the DOM node available at the beginning of the AJAX request was gone when the response come back.
I don't know what you mean by simultaneous, but there are multiple controls on the page that can trigger ajax events.
Any reason that the DOM node would not be available when it comes back?
look like if you are especifying some control's id in rerender and it is not part of the same ajax region.
The problem is because if you specify some id of a control that is not in the same ajax region and the region has the renderRegionOnly property of the set to true, only the content inside the ajax region will be rendered and the response only will have the content inside the ajax region but no the other controls in the page. for that reason is the message ... is not present in reponse