I set the [xyz] and all other JSF Managed Bean and their contained variables to Serializable.
The error went away.
I'm left puzzled as to why the application worked with out the Serializable JSF Managed Bean on someone else's computer but not mine.
After further research, I found my JBoss EAP 6.1 has been configured for Infinispan (http://infinispan.org/), and it was caching the session objects. Because these objects were not Serializable, it couldn't store them and hence throwing the error.
With the old cold, the errors went away as soon as I removed the Inifispan configuration in jboss.