I've been using Docker with the HA configuration as described here: http://blog.keycloak.org/2015/04/running-keycloak-cluster-with-docker.html

I ran into the same problem as David Willson describes in the comments, namely a NullPointerException at org.keycloak.models.sessions.infinispan.initializer.OfflineUserSessionLoader.init(OfflineUserSessionLoader.java:25). Looking at the code, it seems as though a UserSessionPersister was coming back null.

I added to keycloak-server.json the following:

and now everything starts ok. Is this the appropriate fix for a clustered configuration?