Here’s the thing: InterRail official web and app says that it is possible to book seat reservations in Croatia/Hungary. We also got reply from Interrail that yes, those bookings are possible. But it’s not true at all! It is impossible! And you will find many other people having the same issue - there’s always the same message
“Prices unavailable No price results were found for this connection. It is possible that all pass holder seats are sold out. Please try searching for another connection or check 'More information' and try other ways to book this seat reservation. If no pass holder seats are available anymore you need to buy a separate full fare ticket to board” - and it doesn’t matter what dates you choose!
The only option is to book on place… but imagine that it’s high season and you have on other way than book a seat in the same day… Great, isn’t? This is all a one huge lie and make whole trip a nightmare. You risk all that you won’t be able to travel because of that. We will never use interrail again! It’s totally fraud!