The error message shows that the report is failing because of the message:
Could not open file: http://acm.cpha.local/Windchill/servlet/XML4Cognos?table=Site%23Completed_Workflow_Report&uid=&locale=en-us
For some reason, the request to this URL is failing. What does the entry in your apache access log show for this URL? What is the http response code returned?