About session persistence on Tomcat

By default, Tomcat is configured with persistent sessions, which means Tomcat will write to disk all HTTP sessions which are in memory at the time the server is shut down.

Whenever you restart the Tomcat server, it tries to restore the sessions. However, the session contents are meaningful only to the old instance of PolicyCenter, so PolicyCenter throws an exception such as the following:

SEVERE: IOException while loading persisted sessions: java.io.InvalidObjectException: Error
  deserializing Key of "com.guidewire.commons.entity.Key":null
  java.io.InvalidObjectException: Error deserializing Key of "com.guidewire.commons.entity.Key":null
  at com.guidewire.commons.entity.Key.readResolve(Key.java:141)
  ...

Guidewire recommends that you configure Tomcat to disable session persistence.