Chaddington Boomhauer recently alerted me to this JIRA entry, SVC-1509, "Error message when starting group IM session", a phenomenon depicted at left. To judge by the entry, this probelm seems to occur most during chat in large groups, and it's been a recurring issue since 2007. Right now the JIRA entry has just over 1000 users who've voted on it as a priority; to get involved, go here, and log-in with your SL account name/password. Alexa Linden is currently collecting data on the error (read her post on the JIRA discussion, time-stamped 08/Jun/09 02:36 PM), so it is being addressed. Until it's fully resolved, however, perhaps groups may want to consider using third party group messaging systems as an alternative/backup. Suggestions?