JGroups 3.x and 2.x are not compatible; upgrading JBossCache from JGroups 2.6.x to 3.x therefore won't work.
Since JBossCache is old and unsupported, I suggest take a look at Infinispan.
I see. So is that why the latest supported JGroups of jbosscache is 2.6.x?
Thanks for your quick response. I'll see if I'm able to convert CAS to use Infinispan for clustering instead.
Yes, I think so. You would have to change some JBC code to accommodate JGroups 3.x. Nothing difficult, but you'd be changing dead code, so I suggest Infinispan. With mode=repl-xxx, you get the behavior of JBossCache and more.