This reservation can be had at oebb.at.

Fill in the from/to/dates details as if you were booking a normal train ticket, then click ‘Book Ticket’, then on the next screen click ‘Change’ in the box where it says ‘1x Adult’, click ‘Add Discount’ and select ‘Interrail/Eurail Global Pass’, before clicking ‘Confirm’ at the bottom and then ‘Next’ in the top-right-hand corner.

This will take you back to the previous screen - now click ‘Find Services’, then ‘One-way Services’. Select the correct train which should now display only a nominal fee of €3, then scroll down to change the accommodation option from ‘Seat Reservation’ to ‘Sleeper Cabin’. Then add to basket.

Thank you. Very clear. The only problem encountered is that when you book a sleepcabin and you pay, they announce you, after you have payed,  that there is no cabin available. It's an inverse world!

That is strange. You'd better contact ÖBB. You can also book via phone.

In 2018 we tried to reserve a compartment in the sleepingcar (Wien-Bukarest, Dacia) with interrail. Not possible, however normal bookings were possible. I called ÖBB: there was only one or two compartments bookable for interrail. That was the contingent. There were still many compartments left for normal tickets. So, we were a little angry!! I do not know the situation at this moment!

However, the site says that there are no cabins for two persons available at all!! I presume that there is something wrong with this train.

Did ÖBB say anything about that?

Best to contact ÖBB via Twitter. By mail they usually respond very slow.