Hi Milosz,
Thanks for your reply.
Today morning I have re-setup even though I delete/lost the corresponding test run log with the below steps.
1. I have reinstalled LAVA server (#sudo apt-get purge --auto-remove lava-server  and  #sudo apt-get install lava-server) 
2. reinstalled dispatcher at worker side (#sudo apt-get purge --auto-remove lava-dispatcher and #sudo apt-get install lava-dispatcher) 
3. Remove the device and corresponding execution logs.
4. Remove worker
5. Add worker and device again.
I will try to start my debug from the "lava_scheduler_app" log as you said to identify the issue when I face the issue next time.
Is this the lava_scheduler_app log you are referring to i.e  "/var/log/lava-server/lava-scheduler.log" ?
Regards,
Koti