Pb during http session replication : "Problem accessing session data : class java.lang.IllegalStateException metadata is null"

I have a basic JBoss cluster with two nodes that are running on a single server.

Both nodes are up and OK :

• node 1 :
  17:22:29,564 INFO [RPCManagerImpl] Received new cluster view: [172.30.4.76:30577|1] [172.30.4.76:30577, 172.30.4.76:29898]
  17:22:29,572 INFO [LegacyStateTransferIntegrator] Using version 4096
  17:22:29,591 INFO [RPCManagerImpl] Cache local address is 172.30.4.76:29898
  17:22:29,591 INFO [RPCManagerImpl] state was retrieved successfully (in 176 milliseconds)
  17:22:29,613 INFO [ComponentRegistry] JBoss Cache version: JBossCache 'Cascabel' 3.1.0.GA
  17:22:29,680 INFO [Http11Protocol] Starting Coyote HTTP/1.1 on http-172.30.4.76-8080
  17:22:29,767 INFO [AjpProtocol] Starting Coyote AJP/1.3 on ajp-172.30.4.76-8009
  17:22:29,791 INFO [ServerImpl] JBoss (Microcontainer) [5.1.0.GA (build: SVNTag=JBoss_5_1_0_GA date=200905221634)] Started in 1m:20s:161ms

• node 2 :
  17:22:27,152 INFO [RPCManagerImpl] Received new cluster view: [172.30.4.76:30577|0] [172.30.4.76:30577]
  17:22:27,153 INFO [RPCManagerImpl] Cache local address is 172.30.4.76:30577
  17:22:27,159 INFO [RPCManagerImpl] state was retrieved successfully (in 2.02 seconds)
  17:22:27,184 INFO [ComponentRegistry] JBoss Cache version: JBossCache 'Cascabel' 3.1.0.GA
  17:22:27,250 INFO [Http11Protocol] Starting Coyote HTTP/1.1 on http-172.30.4.76-8180
  17:22:27,293 INFO [AjpProtocol] Starting Coyote AJP/1.3 on ajp-172.30.4.76-8109
  17:22:27,314 INFO [ServerImpl] JBoss (Microcontainer) [5.1.0.GA (build: SVNTag=JBoss_5_1_0_GA date=200905221634)] Started in 1m:23s:438ms
  17:22:29,560 INFO [RPCManagerImpl] Received new cluster view: [172.30.4.76:30577|1] [172.30.4.76:30577, 172.30.4.76:29898]

I've deployed tomcat servlet examples and I'm using in particular "Session example" servlet to test http replication

I'm adding attributes in the session that is managed on a node : OK
17:31:04,168 INFO [[/TomcatServletExamples]] SessionListener: sessionCreated('ghh7gRljrScVi+rGT-T-LQ__.ESAJBOSS3_jboss_172.30.4.76_1')
17:31:13,629 INFO [[/TomcatServletExamples]] SessionListener: attributeAdded('ghh7gRljrScVi+rGT-T-LQ__.ESAJBOSS3_jboss_172.30.4.76_1', 'aa', 'aa')
17:31:16,963 INFO [[/TomcatServletExamples]] SessionListener: attributeAdded('ghh7gRljrScVi+rGT-T-LQ__.ESAJBOSS3_jboss_172.30.4.76_1', 'bb', 'bb')

I stop this node : OK
17:31:36,952 INFO [GroupMember] org.jboss.messaging.core.impl.postoffice.GroupMember$ControlMembershipListener@35b83ac6 got new view [172.30.4.76:10321|2] [172.30.4.76:55196], old view is [172.30.4.76:10321|1] [172.30.4.76:10321, 172.30.4.76:55196]
17:31:36,953 INFO [GroupMember] I am (172.30.4.76:55196)
17:31:36,954 WARN [NAKACK] 172.30.4.76:55196] discarded message from non-member 172.30.4.76:10321, my view is [172.30.4.76:10321|2] [172.30.4.76:55196]
17:31:36,966 INFO [GroupMember] Dead members: 1 ([172.30.4.76:10321])
17:31:36,966 INFO [GroupMember] All Members : 1 ([172.30.4.76:55196])
17:31:38,156 INFO [RPCManagerImpl] Received new cluster view: [172.30.4.76:10321|2] [172.30.4.76:55196]
17:31:38,156 WARN [NAKACK] 172.30.4.76:55196] discarded message from non-member 172.30.4.76:10321, my view is [172.30.4.76:10321|2] [172.30.4.76:55196]
17:31:38,205 WARN [NAKACK] 172.30.4.76:55196] discarded message from non-member 172.30.4.76:10321, my view is [172.30.4.76:10321|2] [172.30.4.76:55196]
17:31:38,206 INFO [DocsPartition] New cluster view for partition DocsPartition (id: 2, delta: -1) : [172.30.4.76:1199]
17:31:38,211 INFO [DocsPartition] I am (172.30.4.76:1199) received membershipChanged event:
17:31:38,212 INFO [DocsPartition] Dead members: 1 ([172.30.4.76:1099])
17:31:38,212 INFO [DocsPartition] New Members : 0 ([])
17:31:38,212 INFO [DocsPartition] All Members : 1 ([172.30.4.76:1199])

I'm adding a new attribute : OK (request is served by second node of the cluster) :
17:32:20,789 INFO [[/TomcatServletExamples]] SessionListener: attributeAdded('ghh7gRljrScVi+rGT-T-LQ__.ESAJBOSS4_jboss_172.30.4.76_1', 'cc', 'cc')

I'm adding a new attribute : KO
17:33:09,053 WARN [SessionBasedJBossCacheService] Problem accessing session data : class java.lang.IllegalStateException metadata is null
17:33:09,071 INFO [[/TomcatServletExamples]] SessionListener: sessionCreated('RVmtXKGKvGGWuGjXKxoXNw__.ESAJBOSS4_jboss_172.30.4.76_1')
17:33:09,078 INFO [[/TomcatServletExamples]] SessionListener: attributeAdded('RVmtXKGKvGGWuGjXKxoXNw__.ESAJBOSS4_jboss_172.30.4.76_1', 'dd', 'dd')

=> A new session is created and old values stored in the first session are lost

Do you think it's a bug or did I make a mistake somewhere ?