This is a common error and can be resolved by contacting customer service and have them correct the date of birth they have on record, which is either different from what you're entering or unknown.

Contacting customer service directly can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Note that it can take a few days to receive a reply if you contact them online.

Thank you, Tochjo,

Thank you, i will try do that.

If it is an ungoing problem. NS should solve this problem quickly. NS instead does not do anything at all. 

It's not just an error, it's partly due to the fact that people in the past entered a wrong birthdate. Either by mistake or deliberately.

It’s an error because recently, birthdates went missing or changed in the NS system. In some cases, https://www.ns.nl/mijnns#/persoonsgegevens will even show the correct date but ordering a subscription gives an error.

Deliberately entering a wrong date when creating a Mijn NS account will not help (much), not now and not in the past.

Entering a wrong birthdate when applying for a chipcard however will.