Does the message below mean that I need to flush the user app cache?

17:00:57,923 ERROR [JBossCacheService] externalizeSession(): exception
occurred externalizing session SessionBasedClusteredSession[id:
hBBJL6PzpdiLIYT--PTzhQ** lastAccessedTime: 1197756050714 version: 8
lastOutdated: 0]

I logged in as admin, went under cache and clicked on flush cache.
However, I am running user app in a clutered environment, does that mean I
need to do it on both servers?