Jboss Passivates instances of ur session bean and its not finding the beans when it needs to activate it.
I have seen a similar error being posted in the EJB forums. Try a search on that newsgroup.
Try increasing the number of session beans in memory or tweak the container-configurations. That might help.
Also you might want to add some logging to your ejbPassivate to see how long the session has been passivated before this happens (ie. has the scavenger thread already removed the passivated session from disk before you try to activate it again.)
Thanks, where can I configure the maximum number of session beans? In standardjboss.xml?
Override the container-configuration in your jboss-xml and change the value of "max-bean-life" in the container-cache-conf section.
this is by default 1800. Increase this to a higher value and check if the error still persists.
Look in LRUStatefulContextCachePolicy.java for how this is removed from cache.