Seam portlet throws exceptions in a clustered EPP environment

Issue

We are seeing exceptions like the following every time a request (GET/POST) is made on the portal:

ERROR  [org.jboss.web.tomcat.service.session.JBossCacheService] IOException occurred marshalling value
java.io.NotSerializableException: javax.portlet.PortletMode
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
        at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
        at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
        at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
        at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
        at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
        at java.util.HashMap.writeObject(HashMap.java:1000)

and

ERROR [CacheMarshaller300] Error while marshalling object: ReplicateCommand{...} 
java.io.NotSerializableException: org.jboss.web.tomcat.security.login.WebAuthentication 
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156) 

The portlet has been created using Seam and JBoss Portlet Bridge 2. We only see this in a clustered environment