an instance of class com.atlassian.jira.portal.PortalPageConfigurationImpl is put onto the session at some point. since this class isn't Serializable, the session can't be written to a store.

I suppose it's possible as well that rather than a PortalPageConfigurationImpl being directly saved on the session, some other object is stored there, and the PortalPageConfigurationImpl is reachable from that object.