Welcome to the NS Community!

If you cancel within 14 days, I guess these costs will be reimbursed.

https://www.ns.nl/en/customer-service/termination-cancellation/cooling-off-period.html

After invoking your right of withdrawal, you are entitled to full restitution of your season ticket costs, which will be paid into your bank account within 14 days.

Unfortunately, the administration costs will not be refunded. I was told this by the support team.

Then the support team was wrong. According to Dutch law all costs, including administration costs must be refunded.