I have been on this train last week and i reserved in Germany for 6 bed couchettes for 13,40€.
But in Belgrad there was no rail car with my number or even with couchettes, because the train consisted of one sleeping car and one seat car.
But i could get a place in a triple sleeper for only 10 €. (both prices with IR)
And also i was told at the counter that the reservations (for this train) are only the half in Belgrad compared to Germany.
So maybe you should get your reservation rather here.