These errors fill the log. It seems to occur every time a session expires.

2005-09-01 18:28:53,044 ERROR [org.jboss.web.tomcat.tc5.sso.TreeCacheSSOClusterManager:546] caught exception removing session s+oAvrCQGFcafsVmEaDjwg** from SSO id 36AF2DDAB10613EF37AA470571CF8542
javax.transaction.NotSupportedException: Transaction already active, cannot nest transactions.
at org.jboss.tm.TxManager.begin(TxManager.java:193)
at org.jboss.tm.usertx.client.ServerVMClientUserTransaction.begin(ServerVMClientUserTransaction.java:110)
at org.jboss.web.tomcat.tc5.sso.TreeCacheSSOClusterManager.removeSession(TreeCacheSSOClusterManager.java:509)
at org.jboss.web.tomcat.tc5.sso.ClusteredSingleSignOn.removeSession(ClusteredSingleSignOn.java:669)
at org.jboss.web.tomcat.tc5.sso.ClusteredSingleSignOn.sessionEvent(ClusteredSingleSignOn.java:303)
at org.apache.catalina.session.StandardSession.fireSessionEvent(StandardSession.java:1547)
at org.jboss.web.tomcat.tc5.session.ClusteredSession.expire(ClusteredSession.java:411)
at org.jboss.web.tomcat.tc5.session.ClusteredSession.expire(ClusteredSession.java:345)
at org.apache.catalina.session.StandardSession.isValid(StandardSession.java:567)
at org.jboss.web.tomcat.tc5.session.JBossCacheManager.processExpires(JBossCacheManager.java:665)
at org.jboss.web.tomcat.tc5.session.JBossManager.backgroundProcess(JBossManager.java:662)
at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1283)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1568)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1577)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1577)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1557)
at java.lang.Thread.run()V(Unknown Source)
2005-09-01 18:28:53,045 ERROR [org.jboss.web.tomcat.tc5.session.JBossCacheManager:692] processExpires: failed with exception: org.jboss.util.NestedRuntimeException: TreeCacheAop.endTransaction(): ; - nested throwable: (java.
lang.NullPointerException)