The obvious one is Vienna - Bucharest as it’s a very long journey otherwise + it’s cheap. I see it available on OBB: add Interrail as a discount and select one-way tickets. 84€ for a private sleeper compartment = ideal!

Book as soon as your dates are set as it’s a popular route.

Btw arrival is at 15:06 and not 14:06 as OBB claims (their planner can’t cope with time zones).

You may go by night train directly from Rotterdam (21.22) to Praha: The Good Night Train to | European Sleeper.

Then by day train from Praha to Wien (short journey; many choices).

Then by night train from Wien (19.42) to Bucuresti: Angebote & Tickets (oebbtickets.at).

It’s also possible to do the whole trip by night trains when going from Praha (22.03) to Budapest and from there into Romania. There’s even a much better choice from Budapest if you do like vintage sleeper with windows to open and a proper restaurant car for dinner and breakfast.

Thanks! We have by now been able to make reservations from Rotterdam to Bucharest via Vienna! Two night trains. 

Now trying to figure out how to book a reservation from Bucharest to Prague. It turns out that the Bucharest train service does not allow for only seat reservation, except at the train station. However, in high season that doens't seem to be a good idea, waiting only 2 weeks ahead to reserve. We would like to take a night train with couchette. 

2 weeks ahead shouldn't be an issue. You may also try at German or Austrian ticket counters.

Otherwise get a full fare ticket and save the pass day.