I suppose you got an e-ticket to Brussels? In that case you could have filled in a reduction for the Netherlands of 40 percent, assuming you traveled after nine o’clock in the morning. In that case there is no need to check in. The fine is not correct. You should have shown your e-ticket, did you?

For the 50 euro fine you do need to contact Customer Service. Do hold your international e-ticket at hand. €37,40 is the correct fare with 40 percent reduction in the Netherlands. Your ov-card has to be shown to prove your right of reduction. Is your subscription for 40 percent reduction valid? If right, no fine, if not, you will have a fine, though.

The only solution for this is to contact the customer service, we can't help you at this community.

However, the described situation appears weird to me: do I read correctly you’re travelling on an E-ticket with 40% discount for the Dutch part, and got the 50 euro correction fare when showing your chipcard to the conductor as proof of being eligible for the discount?

Checking in should not be required when travelling on an international E-ticket. But in case you've used an E-ticket with 40% discount before 9.00 hrs, the 50 euro correction fare is for travelling with an invalid ticket and that you cannot correct anymore.

The 50 euro correction fee happens when a conductor checked your ov-chipcard for the 40% reduction, and (mistakenly) registered the missing check-in.

This happens a lot (typically with NS Flex) and should be corrected, provided you travelled after 9.00 on a weekday.