Please check on tickets.oebb.at first : add Interrail/Eurail as a discount. nightjet.com doesn't work that well for EN (Euronights) : Stuttgart/Zurich - Zagreb is one of them.

Interrail/Eurail has become even more unreliable recently so don't use that website.

If you have other questions, mention the route and we'll have a look. :)

Thankyou for you answer! I already checked the Oebb website. But you said add Interrail as a discount? What do you mean by that? 

For trains with compulsory reservations on ÖBB 

you select ONE WAY TICKETS (NOT only a seat reservation!) and add Interrail as discount card. So you get the correct fare for Interrail (in this case a reservation for nighttrain) 

I know a bit strange but the website works like this.

Ah in that way! Thankyou but sadly its still not avaible to buy tickets for the reservation :( 

It says that there are planned construction works on the route. That’s why the timetable is not yet released. Keep checking regularly. There will be for sure seats at least available. I have used this route a few times and often had the compartment for myself or shared with one other person…

Which route and exact date please ? We can't give precise advice or suggest alternatives otherwise.

As said the booking may not be possible if there are engineering works on the route.

As she is from the Netherlands I suppose it is Stuttgart-München-Salzburg-Zagreb

Oops didnt understand that! But thankyou very much for the answer, now i feel a little bit better and less stressfull!

I'm not sure because OP spoke about nightjets and there is one leaving the Netherlands towards Austria too.

Its the route who leaves by 20:29 from Stuttgart and i have to step out on 9:16 in Zidani. And that says indeed that its under construction.

Also true! I just read Stuttgart-Zagreb in your first comment and checked that one.

but now also checked the Nightjet and that one is also not yet open for booking it seems (Amsterdam-Rosenheim)