Take a look at bug 1057188.

Having BPMS cluster with two EAP nodes and deployment unit with PER_PROCESS_INSTANCE session strategy, the exception "java.lang.IllegalStateException: Invalid session was used for this context org.kie.internal.runtime.manager.context.ProcessInstanceIdContext@2deb02a5" is still present on the second node in the moment of its shutdown.

See the attached stacktrace.