Hi Vahaj,
please read carefully the stack trace you've posted here and then you'll see something relevant to investigate:
for example this could be something relevant:
"Caused by: org.activiti.engine.ActivitiException: problem evaluating script: sun.org.mozilla.javascript.internal.EcmaError: ReferenceError: "ExecuteScriptOnTaskCompleteBpmnParseHandler" is not defined. (#33) in at line number 33"
the reason could be: "ExecuteScriptOnTaskCompleteBpmnParseHandler" is not defined.
Did you understand the script you want to execute in your system?