Stacktrace from EAP 4.2 CP05:
14:08:33,085 ERROR [JBossCacheManager] Problem rolling back session mgmt transaction
java.lang.NullPointerException
at org.jboss.web.tomcat.service.session.JBossCacheManager.loadSession(JBossCacheManager.java:1071)
at org.jboss.web.tomcat.service.session.JBossCacheManager.findSession(JBossCacheManager.java:832)
at org.apache.catalina.connector.Request.doGetSession(Request.java:2266)
at org.apache.catalina.connector.Request.getSession(Request.java:2066)
at org.jboss.web.tomcat.service.session.JvmRouteValve.checkJvmRoute(JvmRouteValve.java:91)
at org.jboss.web.tomcat.service.session.JvmRouteValve.invoke(JvmRouteValve.java:81)

The NullPointerException is when referencing the transaction manager in loadSession:
if(tm.getTransaction() == null)

When it has been cleared during a previous call to stop():
tm = null;