Symptoms
Invalid high surrogate character (0xDC09) error message when try to open a form to process
Diagnoses
It seems as though the processInstance.ViewFlow is causing a issue when K2 are passing in a SourceCode.Workflow.Client.ProcessInstance to a FromApi function. The error occurs when the worklistitemcollection get serialized with xmlserializer.
Resolution
Please contact support for the relevant coldfix with regards to this issue.