$200 seems quite reasonable for digging through the files and trying to figure out what happened.

You didn't tell us the arrangement with your ex in relation to the mortgage and house. If I were to guess, you were both on the mortgage but he took the house in the divorce and was supposed to make payments. You probably didn't include the mortgage in the Chapter 13 plan, as he was supposed to make the payments. There was no discharge of your responsibility for the debt, as it was a restructuring not a Chapter 7. And now there's an outstanding balance, no discharge, and your name's on the mortgage so they want you to pay.

If that's right, they would be just as happy if your ex- paid, so if you have recourse under your divorce judgment perhaps you can get the divorce court to order him to pay the balance due.