If you experience such an error it is very well based on having the JBoss server and/or the clients started using JDK 6.0. Some more explanations can be found in JBoss Forum topics here and here.
Sometimes I run in this problem as my production systems still use JBoss 4.0.5 GA. Beginning with JBoss 4.2.0GA this error should not appear anymore. Sometime in future I will update my servers. Until then, I will stick with JDK 1.5.0.