I checked out the source code of JBoss Serialization and I've attached the code to my JBoss installation.
Problem seems to be related to a property ResourceBundle which is in my OneaSessionBean. Passivation fails on this property, because this object cannot be serialized (http://docs.oracle.com/javase/6/docs/api/java/util/ResourceBundle.html). When I mark it as "transient", the problem seems to be resolved!
If you want to know how I got to this solution, please read the following blogpost: http://www.jochus.be/site/2012-08-06/java/debugging-passivation-errors-in-jboss