If your debtor protests the invoice within the statutory period of one month and 8 days following the reminder, then the procedure via Unpaid will stop at that point. You will be billed for the costs effectively incurred by the bailiff. However, you can still attempt to recover these costs, along with the rest, if you decide to pursue the claim further in court.
You can always take further steps to attempt to recover your money, but simply no longer via Unpaid. A judge will have to assess whether your debtor's protest is valid. In other words, if you decide to continue, you will need the services of a lawyer. The law firm De Groote – De Man, with which Unpaid collaborates, has experience and expertise in this area.