I got this exception, while working on a Ajax+JSF+Seam project. I ignored it for a while till it came back haunting at the UAT.
A little investigation into the exception and googling help me find the answer. Here is what happened, I had a ajax component which was taking a long time to render the response, in the meanwhile another request was sent to the server through some other component action ex a submit button, this call failed to render the response, reason being the first call had obtained a lock in the render_reponse and hadnt released it, so the second call failed to obtain a lock and hit back with the exception.
You could solve this in one of the following ways,
1. Showing a wait dialog so that the users understand that there is some processing going on.
2. queueing the request, a4j has a component named 'queue' which you can leverage.
If there are better ways to handle this, I will be glad to add it to my kitty.