Hi Clebert, thanks for responding so quickly. I hadn't seen JBREM-649 previously, but it does /appear/ to be the same error. My clients and server stack traces seem to bear this out - the server is trying to do something with an object read out of a HashMap, the client seems like it's trying to write to (what I assume to be) the same HashMap. server: at org.jboss.serial.persister.RegularObjectPersister.readSlotWithMethod(RegularObjectPersister.java:107) client: at org.jboss.serial.persister.RegularObjectPersister.writeSlotWithMethod(RegularObjectPersister.java:120) Is it an easy fix? View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3993453#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...