A maximum-fare charge for a tap-out the traveller is certain they made turns up in bank statements every month, and the amount is large enough to sting. A correction route exists, but it comes with a deadline, so knowing about it before the window shuts is the entire difference.
That's almost certainly OVpay charging you the maximum possible fare because the system never saw you tap out, and it's fixable. Go through OVpay's own correction flow — either in the app, on ovpay.nl, or via uitcheckgemist.nl, which now handles OVpay disputes as well as old OV-chipkaart ones — where you fill in where you actually got on and off and it recalculates the fare and refunds the difference. The window is 60 days from the trip, though you can't file immediately: corrections only open up starting six hours after you traveled. NS caps this at three corrections a year; other carriers use similar limits, so it's not something to lean on constantly. To stop it happening again, check your OVpay trip history the same day you travel — if a check-out is missing, you'll catch it while you still remember exactly which stop you got off at, which makes the correction form much faster to fill in.