That looks like a bug to me, since your pass is valid from Brussels.

In addition, don't use the Interrail reservation service to book optional reservations in/to/via Germany but rather Deutsche Bahn (click on "Seat Only”). Interrail charge 8 € per person per train, whereas DB charge 4 € per person per journey. So for Brussels - Berlin with a change in Cologne, DB will charge 4 € and Interrail 16 €. DB will also let you choose your seat.

You do as such not have to make a REServ, its only optional, but for most times advised as trains can be busy. Also note the still prevailing special German requirements in what type of mask to wear all the time!

Thank you all!

I was right now reserving seats with the deutsche bahn website and just thought of another thing - this german rail pass is a mobile pass. I’m afraid once I’ll activate it, I will have to make a reservation through the interrail pass app anyway, is that right? And then again it won’t work?

What should I do, maybe find a way to change the mobile pass into a paper one?

The pass and reservations are separate. You can make reservations wherever you like.

You should also be able to add a journey with ICE to Brussels to My Trip / My Pass. If that doesn't work, then that is a bug. You could test that if you want and deactivate your pass again (cancel all travel days) after your test.

Woohoow it all worked out! Thanks a lot!