My impression is that, because of the delay in taking into service the new IC carriages on Copenhagen-Hamburg, the timetable is only now being integrated in the planners and reservations are not open yet. So the Interrail website is already showing the timetable (they've finally massively increased their update frequency) but DB and DSB not yet. And nobody can book this train yet.

I suggest to wait a few days and then book this train here:

https://travel.b-europe.com/dsb-rail/en/reservation-only#TravelWish

It's only half the price compared to Interrail (DKK 30=€4 instead of €8). If you also want to book Hamburg-Berlin, use tickets.oebb.at (€3 instead of €8).

Edit: these IC carriages are not exactly new, but DSB rent them from DB and they're facing delays with approval for use in Denmark.

Thank you so much for the reply! 

That link above is the first one I’ve seen where you can book these tickets online (trying to reserve Brussels to Munich and always seems impossible to do seat reservation only). Is there a way to pay in Euros/ pounds?

Many reservations can be booked online; most on multiple websites. This includes Brussels-Munich. For day trains, use bahn.com with "Seat only”. For better advice, please mention date and departure time.

@rvdborgt should not be possible to book (when the booking is open) Berlin-Hamburg-Kopenhagen via bahn.com for 4,50 EUR instead of 3+3 EUR of ÖBB? 

Are there informations if with the “new” trains the compulsory reservations like in other summers is still a thing?

Yes, but only when reservation is optional. DB always charges mandatory reservations separately and they still don't sell them online.

Yes, it's still a thing unfortunately.