Deploying pages fails and the following exception is being recorded in the cd_core.log of the Content Deployer.
 

ERROR [ProcessingAkkaSpringContext-akka.actor.default-dispatcher-3] SqlExceptionHelper - ORA-00001: violation de contrainte unique (XXX.REFERENCE_ENTRIES_BK)

ERROR [ProcessingAkkaSpringContext-akka.actor.default-dispatcher-3] TridionTransactionalExecutableStep - Rollback transaction due to error. ExecutionId: tcm:0-XXX-66560 error: 
javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
	at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:154)
	at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:181)
	at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:188)
	at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1460)a
	at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1440)
	at jdk.internal.reflect.GeneratedMethodAccessor351.invoke(Unknown Source)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:351)
	at com.sun.proxy.$Proxy175.flush(Unknown Source)
	at com.tridion.storage.persistence.JPABaseDAO.flush(JPABaseDAO.java:417)
	at com.tridion.storage.persistence.JPAReferenceEntryDAO.create(JPAReferenceEntryDAO.java:53)
	at com.tridion.storage.services.ReferenceCounter.replaceReferences(ReferenceCounter.java:185)
	at com.tridion.storage.deploy.workers.ReferenceEntryWorker.doDeploy(ReferenceEntryWorker.java:88)
	at com.tridion.deployer.model.transaction.TransactionLogItemWorker.doWork(TransactionLogItemWorker.java:43)
	at com.sdl.delivery.deployer.steps.TridionPreCommitStep.executeWorkers(TridionPreCommitStep.java:156)
	at com.sdl.delivery.deployer.steps.TridionPreCommitStep.process(TridionPreCommitStep.java:107)
	at com.sdl.delivery.deployer.engine.pipeline.actor.StepActor.execute(StepActor.scala:73)
	at com.sdl.delivery.deployer.engine.pipeline.actor.StepActor.com$sdl$delivery$deployer$engine$pipeline$actor$StepActor$$processStep(StepActor.scala:58)
	at com.sdl.delivery.deployer.engine.pipeline.actor.StepActor$$anonfun$receive$1.applyOrElse(StepActor.scala:41)
	at akka.actor.Actor.aroundReceive(Actor.scala:539)
	at akka.actor.Actor.aroundReceive$(Actor.scala:537)
	at com.sdl.delivery.deployer.engine.pipeline.actor.StepActor.aroundReceive(StepActor.scala:27)
	at akka.actor.ActorCell.receiveMessage(ActorCell.scala:614)
	at akka.actor.ActorCell.invoke(ActorCell.scala:583)
	at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:268)
	at akka.dispatch.Mailbox.run(Mailbox.scala:229)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
	at org.hibernate.exception.internal.SQLExceptionTypeDelegate.convert(SQLExceptionTypeDelegate.java:59)
	at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:42)
	at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:113)
	at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:99)
	at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:178)
	at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3176)
	at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3690)
	at org.hibernate.action.internal.EntityInsertAction.execute(EntityInsertAction.java:90)
	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:604)
	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:478)
	at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:356)
	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
	at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1454)
	... 26 common frames omitted

 

Solution: Verify that the Content Deployer is pointing to the right Content Datastore and State Store databases

For this particular situation, a mismatch in the database configuration can lead to deployment failures.
For the Content Deployer, check that in both the deployer-conf.xml and cd_storage_conf.xml files the database elements are pointing to the expected databases.