Error persisting activity state - Unable Start a workflow

Hi guys,

we have an UserApp cluster (4.5.4 Build Revision 42545) with 2 nodes (Tomcat 7.0.55) and a single Postgres DB (9.3.4).
When all UserApp are running, sometime we encounter this error:

2017-07-06 09:33:47,791 [pool-3-thread-6] ERROR com.novell.soa.af.impl.core.ProcessImpl- [RBPM] Process requestId [4d50e0ca755b484598526c3f684bebe8], Id [cn=blablabla,cn=RequestDefs,cn=AppConfig,cn=UserApplication,cn=Driver Set,o=services], activityId [Start]: Error persisting activity state, scheduling activity for retry.
com.novell.soa.af.impl.activity.ActivityPersistenceException: Activity Id [Start]: Error persisting activity state.
at com.novell.soa.af.impl.activity.ActivityNode.persistStatus(ActivityNode.java:582)
at com.novell.soa.af.impl.activity.ActivityNode.process(ActivityNode.java:218)
at com.novell.soa.af.impl.activity.StartActivity.process(StartActivity.java:70)
at com.novell.soa.af.impl.activity.ActivityNode.notifyArrive(ActivityNode.java:206)
at com.novell.soa.af.impl.activity.RunnableActivity.run(RunnableActivity.java:50)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: com.netiq.persist.PersistenceException: com.netiq.persist.PersistenceException: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [com.novell.soa.af.impl.persist.ActivityStatusInfo#requestId = [4d50e0ca755b484598526c3f684bebe8] activityId = [Start]]
at com.netiq.persist.HibernateDAO.saveOrUpdate(HibernateDAO.java:101)
at com.netiq.persist.HibernateDAO.saveOrUpdate(HibernateDAO.java:71)
at com.novell.soa.af.impl.persist.ActivityStatusInfo.update(ActivityStatusInfo.java:271)
at com.novell.soa.af.impl.activity.ActivityNode.persistStatus(ActivityNode.java:565)
... 7 more
Caused by: com.netiq.persist.PersistenceException: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [com.novell.soa.af.impl.persist.ActivityStatusInfo#requestId = [4d50e0ca755b484598526c3f684bebe8] activityId = [Start]]
at com.netiq.persist.HibernateUtil.commit(HibernateUtil.java:249)
at com.netiq.persist.HibernateDAO.commit(HibernateDAO.java:502)
at com.netiq.persist.HibernateDAO.saveOrUpdate(HibernateDAO.java:91)
... 10 more
Caused by: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [com.novell.soa.af.impl.persist.ActivityStatusInfo#requestId = [4d50e0ca755b484598526c3f684bebe8] activityId = [Start]]
at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:1950)
at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2594)
at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:2494)
at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2821)
at org.hibernate.action.EntityUpdateAction.execute(EntityUpdateAction.java:113)
at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:273)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:265)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:185)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:51)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1216)
at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:383)
at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:133)
at com.netiq.persist.HibernateUtil.commit(HibernateUtil.java:247)
... 12 more

Request doesn't start and we lost all info.
If only 1 UserApp is up, everything works fine.
Which configuration can I check in the cluster?

thanks