Symptoms

When attempting to execute the "List Process Instances" method with a system generated workflow reports/solutions SmartObject, the list method fails to execute with the following error message.
"Empty URI is not allowed in WITH XMLNAMESPACES clause"
 

Diagnoses

The issue is related to bad xml schema, the process's xml fields created under Process/Activity Data needs to recreate/re-import to the workflow.
 

Resolution

Using Visual Studio to create an XML schema from standard XML document and importing xml schema back to the workflow resolved the issue.