My experience with really odd API errors that point to odd name fields seems to always be something in the Item Specifics. That's at least a good place to look.
What is the Category for the item(s) and does anything like Pin Code exist as a fill-in field info for that pop-up you get for Choose Item Specifics?
If that's not the situation, I'd enter a Support Case so Auctiva can help identify the exact cause of the error.
Danno