Load testing an application on Websphere 7 with a load of 2400 concurrent users the following exception occurs:

Trace: 2011/05/06 17:40:34.644 01 t=9B64F8 c=UNK key=P8 (13007002)
ThreadId: 0000005d
FunctionName: com.icesoft.faces.webapp.http.servlet.SessionDispatcher$Monitor.shutdown
SourceId: com.icesoft.faces.webapp.http.servlet.SessionDispatcher
Category: INFO
ExtendedMessage: Session already invalidated.
Trace: 2011/05/06 17:40:34.644 01 t=9B64F8 c=UNK key=P8 (00000011)
Description: Log Java Message
Message: Session already invalidated.
Exception in thread "Session Monitor" java.util.ConcurrentModificationException
.at java.util.HashMap$AbstractMapIterator.checkConcurrentMod(HashMap.java:122)
.at java.util.HashMap$AbstractMapIterator.makeNext(HashMap.java:127)
.at java.util.HashMap$ValueIterator.next(HashMap.java:212)
.at java.util.AbstractCollection.toArray(AbstractCollection.java:333)
.at java.util.ArrayList.<init>(ArrayList.java:87)
.at com.icesoft.faces.webapp.http.servlet.SessionDispatcher$Listener$1.run(SessionDispatcher.java:276)
Trace: 2011/05/06 18:09:48.887 01 t=9B93E8 c=UNK key=P8 (13007002)
ThreadId: 00000055
FunctionName: com.ibm.ws.runtime.component.ThreadMonitorImpl
SourceId: com.ibm.ws.runtime.component.ThreadMonitorImpl
Category: WARNING
ExtendedMessage: BBOO0221W: WSVR0605W: Thread "WebSphere:ORB.thread.pool t=009af5d0" (00000025) has been active for 710189 millise
conds and may be hung. There is/are 1 thread(s) in total in the server that may be hung.