Cannot find execution with id for external Task


Hi Camunda Team,

I am having a load of around 2000 process Instance with camunda version 11.
We have an external task implementation for the service task.
I am having 2 camunda nodes running with 5 external Task nodes.
maxTask = 1
lockDuration = 60000
After a certain time period I am facing this exception in my camunda engine
2019-08-13 13:53:40.065 WARN 1 — [nio-8080-exec-1] ExceptionHandler : org.camunda.bpm.engine.exception.NullValueException: Cannot find execution with id 9541e9d7-bdd1-11e9-bc26-7ea93146e9a9 for external task 9541e9d9-bdd1-11e9-bc26-7ea93146e9a9: execution is null
Please find attached file for complete logs for camunda engine and external Task client.
Timestamp = 2019-08-13 13:53:40.068
logs-from-camunda-in-camunda-deployment-58b4c968d-tlzk8.txt (48.6 KB)logs-from-create-task-plumbing-in-create-task-plumbing-deployment-86d886f97b-82wc8.txt (456.9 KB)

Don’t get any clue for this exception.
And in plumbing we get 500 response status because of this.
Is this issue occurring because we have more external task client comparing to camunda engine. So same time another worker get’s serverd for the same execution id?
How can we prevent this?
Any best practice or solution?

Thanks in Advance