I've tried to export the structure of a tracker in XML format, in order to import it into another project, but the resulting Tracker_sla.xml file was reported as incorrect: on line 906 of attached Tracker_sla.xml file, the <field_id REF=""/> XML tag is missing a value for attribute REF.

For testing purposes, I've tried to copy/paste such a value from another (similar) line, without worrying about the meaning of that value, and then the import was successful.