Workflow fails to start with "Error in expanding variable parameter"

Symptom

When trying to start a workflow in the workflow manager, you get in the integration service pane at the bottom:

Error in expanding variable parameter Please check the Integration Service log for more information.

Cause

You have a parameter file and tried to do sort of a self assignment like

$PMSourceFileDir=$PMSourceFileDir/some/path

Cure

Programming language usually support such things but PowerCenter parameter do not. Create a work around.