ClassCastException on concurrent EJB3 method invocation from different beans

Two different EJB3 stateless session beans A and B packaged into separate EAR files deployed on the same JBoss instance
invoke a method of another stateless session bean C on another JBoss instance that returns an object of a user
defined class Value.
There seems to be a multi-threading issue.
Sometimes when both beans concurrently call the method of C an ClassCastException Value is thrown within the remote
interface proxy before the result object is returned to the calling bean.
The class Value and the remote interface for C are packaged into a JAR file within each EAR file inside a "lib" folder.

If this JAR file is deployed instead and not packaged into the EAR files of bean A and B everything works fine.