do not point directly to the id of modal panel. Point to the id of any inner component.
thanks for your reply.
I tried too to use a outputpanel around the modalpanel, i had the same problem. So that means we should never refresh a zone in which there is a modalpanel ?
the strange thing is it occurs when a rich:tree present in the page.
Sergey means to use output panel not around but nested to modal panel to refresh just content but not the whole panel.