Now this sounds like that request does not include conversation Id.
See if you can add <f:param/> with conversation Id. I also guess that <rich:paint2D/> does the second request as GET, am I right?
Yes, that's absolutely right. I didn't think about that, but <rich:paint2D/> drops the page's request parameter(s), so the request (which is indeed a GET request) does not include the conversation id.
Unfortunately, adding an <f:param/> doesn't work. Any alternatives? :-/