http://blogs.msdn.com/jgalla/archive/2008/08/08/scaling-out-the-viewer-control-and-rsexecutionnotfound.aspxOne of the criteria that the report server uses to match the provided SessionID with a stored report is that the SessionID has to be provided by the same user that initially created the session. Usually, this is the case. Someone browses the report inen-USCommunityServer 2.1 SP1 (Build: 61025.2)