Overpayment letter from the unemployment office - 5 months and counting

The agency says I reported earnings incorrectly, but I think I followed the instructions. Any experience at all would help. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.