Try https://shop.oebbtickets.at/en/ticket (add Interrail/Eurail as a discount and select one-way tickets). A private double compartment should be 84€ → a pretty good deal compared to other night trains!

Thank you for your answer, Thibcabe.

Following your instructions, I was able to see this offer. But, as I've already paid my Eurail pass, this offer would mean I would pay another time the train ticket and not the bed reservation only. For my boyfriend, whose Eurail app works properly, a bed reservation costs between 15 € and 40 € (depending on the chose configuration).

We bought the same passes, on the same day. This situation makes no sense to me…

Is there another way to solve this problem ?

One of you should make the reservations for both of you. And the reservations are not connected to your pass. 

Make a reservation for the 2 of you following the instructions from @thibcabe above. 

Note that the app doesn't make any reservations. 

84€ is the passholder fare for 2 ppl in a private compartment (so 42€ each). If you add Interrail/Eurail as a discount it will show the adapted price. Otherwise a ticket costs 150€ each for that one.

A seat with the pass would be 3€ but I do not recommend that at all. Of course you could travel in a shared couchette compartment for 13.40-20€ but for an 18h journey (+ delay) a private compartment makes a lot of sense.

Do not trust anything coming from the Rail Planner app. It is unreliable, shows wrong data, it’s missing trains, etc. It’s also updated twice a month only.

Okay, I finally got what you meant Thicabe ! Thank you for your precious advice. (Thank you to AnnaB also )

Glad it worked! Only note that OBB struggles with timezones so the arrival is supposed to be at 15:06 (local time). Expect to be woken up by border guards but it’s part of the experience haha :)