This is a real stack trace we obtained when running into this issue:

org.hibernate.AssertionFailure: Unable to perform beforeTransactionCompletion callback
at org.hibernate.engine.spi.ActionQueue$BeforeTransactionCompletionProcessQueue.beforeTransactionCompletion(ActionQueue.java:952)
at org.hibernate.engine.spi.ActionQueue.beforeTransactionCompletion(ActionQueue.java:521)
at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2452)
at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:473)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:156)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$100(JdbcResourceLocalTransactionCoordinatorImpl.java:38)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:231)
at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:68)
at org.springframework.orm.hibernate5.HibernateTransactionManager.doCommit(HibernateTransactionManager.java:625)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:746)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:714)
at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:533)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:304)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
at org.springframework.security.access.intercept.aopalliance.MethodSecurityInterceptor.invoke(MethodSecurityInterceptor.java:69)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:212)
at com.sun.proxy.$Proxy158.save(Unknown Source)
at com.renxo.cms.integration.CustomerFactoryImpl.getCustomerForSubscriberNumberInternal(CustomerFactoryImpl.java:570)
at com.renxo.cms.integration.CustomerFactoryImpl.getCustomerForSubscriberNumber(CustomerFactoryImpl.java:212)
at com.renxo.cms.web.struts.action.customer.CustomerAction$BatchBlacklistTask.run(CustomerAction.java:649)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: javax.persistence.PessimisticLockException: could not execute batch
at org.hibernate.internal.ExceptionConverterImpl.wrapLockException(ExceptionConverterImpl.java:249)
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:103)
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:157)
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:164)
at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1443)
at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1423)
at org.hibernate.envers.internal.synchronization.AuditProcess.doBeforeTransactionCompletion(AuditProcess.java:177)
at org.hibernate.envers.internal.synchronization.AuditProcessManager$1.doBeforeTransactionCompletion(AuditProcessManager.java:47)
at org.hibernate.engine.spi.ActionQueue$BeforeTransactionCompletionProcessQueue.beforeTransactionCompletion(ActionQueue.java:946)
... 24 more
Caused by: org.hibernate.PessimisticLockException: could not execute batch
at com.renxo.cms.dao.hibernate.CustomOracleSQLExceptionConversionDelegate.convert(CustomOracleSQLExceptionConversionDelegate.java:105)
at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:42)
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:111)
at org.hibernate.engine.jdbc.batch.internal.BatchingBatch.performExecution(BatchingBatch.java:128)
at org.hibernate.engine.jdbc.batch.internal.BatchingBatch.doExecuteBatch(BatchingBatch.java:104)
at org.hibernate.engine.jdbc.batch.internal.AbstractBatchImpl.execute(AbstractBatchImpl.java:147)
at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.executeBatch(JdbcCoordinatorImpl.java:212)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:629)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:474)
at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:337)
at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1437)
... 28 more
Caused by: java.sql.BatchUpdateException: ORA-08177: can't serialize access for this transaction

at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:12296)
at oracle.jdbc.driver.OracleStatementWrapper.executeBatch(OracleStatementWrapper.java:246)
at com.zaxxer.hikari.pool.ProxyStatement.executeBatch(ProxyStatement.java:128)
at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeBatch(HikariProxyPreparedStatement.java)
at org.hibernate.engine.jdbc.batch.internal.BatchingBatch.performExecution(BatchingBatch.java:118)
... 36 more