Per email from Guus:
We quite often upgrade the domain at igniterealtime.org with a new Openfire build. Even when no cluster-related code has changed, we frequently get errors - notably on the admin panel page that displays the current user sessions. Stack traces will appear there, complaining about the unavailability of a particular cluster node identified by some byte array. The issue disappears after restarting both cluster nodes, while making sure that at some point in time all nodes are shut down.