You have to book via Snälltaget Website (search for the train and than use as discount code “Interrail”. I would suggest to try to not book via Interrail Reservation service because it doesn’t work most of the time and it cost more. For EC6 you can book directly via bahn.de for 4€ a seat. 

Thanks, I´ve booked it at the Snälltaget Website. 

For the EC I only need a “seat reservation” (if i want it) and than i have to show my interrail-app Global Passes in a ticket control?

Yes, for the EC in Germany reservations are optional.

You show the QR code that you create for the journey in the app in the ticket control. And of course any seat reservation if applicable. 

and with a “Global Pass First Class” i can travel in an EC or ICE without reservation? (Replys in German are okay, too! )

No matter if you have a 1st or 2nd class pass you can travel in EC or ICE in Germany without reservation. If you are a German resident you only have 2 traveldays, outbound and inbound, in Germany though. 

In der 1. Klasse in Deutschland macht es sinn zu reservieren, weil die Tickets der Deutschen Bahn in der 1. Klasse eine Gratis Reservierung haben und damit alles Reserviert sein kann. 

Hi @Michael S.  i recommend to take the 2hours earlier EC from Osnabrück to Hamburg

This train offer in 1st class a panoramic carriage :)

and you have enough time in Hamburg for the case of delay´s :/