I am looking to book a ticket from Budapest to Bucharest on board the 473 Ister train operated by Hungarian railway (https://www.mavcsoport.hu/en) departing Dec 6th 2018. The website mentions that bookings open 90 days in advance but it does not list any tickets for Dec 6th (the last date for which tickets are available are Dec 5th).
It has been this way for quite a few days now. It is highly unlikely that the entire train is sold out for this date (or for any dates post Dec 5th since they all show tickets not available)
I have sent an email to Hungarian Railway asking them the cause and an automated reply that I got back said it takes anywhere from 1-3 months for them to get back.