OptimisticLockingException in the process after completing a user task

Hello all,
I’m getting OptimisticLockingException in the process after completing a user task.
The occurrence of this exception is very rare or I couldn’t find any pattern.
I searched around this exception and found that it may occur due to parallel execution going into race condition but I have no parallel gateway in my process.
Can anyone tell me what could be the possible reasons of this and how to solve this?
Stacktrace :

WARN [org.camunda.bpm.engine.jobexecutor] (pool-10-thread-23) ENGINE-14006 Exception while executing job ab83aef3-8d0b-11e7-87cc-541320524153: : org.camunda.bpm.engine.OptimisticLockingException: ENGINE-03005 Execution of ‘DELETE TimerEntity[ab83aef3-8d0b-11e7-87cc-541320524153]’ failed. Entity was updated by another transaction concurrently.
at org.camunda.bpm.engine.impl.db.EnginePersistenceLogger.concurrentUpdateDbEntityException(EnginePersistenceLogger.java:125)
at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.handleOptimisticLockingException(DbEntityManager.java:323)
at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flushDbOperationManager(DbEntityManager.java:295)
at org.camunda.bpm.engine.impl.db.entitymanager.DbEntityManager.flush(DbEntityManager.java:278)
at org.camunda.bpm.engine.impl.interceptor.CommandContext.flushSessions(CommandContext.java:247)
at org.camunda.bpm.engine.impl.interceptor.CommandContext.close(CommandContext.java:176)
at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:113)
at org.camunda.bpm.engine.impl.interceptor.JtaTransactionInterceptor.execute(JtaTransactionInterceptor.java:58)
at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:66)
at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:30)
at org.camunda.bpm.engine.impl.jobexecutor.ExecuteJobHelper.executeJob(ExecuteJobHelper.java:35)
at org.camunda.bpm.engine.impl.jobexecutor.ExecuteJobHelper.executeJob(ExecuteJobHelper.java:28)
at org.camunda.bpm.engine.impl.jobexecutor.ExecuteJobsRunnable.executeJob(ExecuteJobsRunnable.java:82)
at org.camunda.bpm.engine.impl.jobexecutor.ExecuteJobsRunnable.run(ExecuteJobsRunnable.java:56)
at org.jboss.threads.SimpleDirectExecutor.execute(SimpleDirectExecutor.java:33)
at org.jboss.threads.QueueExecutor.runTask(QueueExecutor.java:808)
at org.jboss.threads.QueueExecutor.access$100(QueueExecutor.java:45)
at org.jboss.threads.QueueExecutor$Worker.run(QueueExecutor.java:849)
at java.lang.Thread.run(Thread.java:745)
at org.jboss.threads.JBossThread.run(JBossThread.java:320)```

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