This internal error message is generated if QuarkXPress notices that in a chain of linked boxes, one of the linked boxes is linked to, or from, the message title will say, a non-slugged textbox. In principle, this is impossible, as all textboxes are slugged (marked by Q++ for later retrieval) during generation.
This message is thus an indication that the QuarkXPress document corresponding to present message is most likely corrupted. You should try to copy it into a fresh file and try again.
If the problem remains, please send the script to technical support, being sure to mention the error message ID listed at the bottom of this topic.