The default converter check that the events and data/source parameters are serializable before sending them. Problem is that XWikiContext is declared as Serializable which is far from being true.
Not very nice for the log but not a big deal since in this case it just fail to send the event to the cluster which is exactly what we want. Need to find a way to skip XWikiContext and other classes in the same situation. The main issue is that anything can be declared as Serializable even if not true.