Problem with registration of Paper Pass

Where are you seeing that message? Could you post a screenshot?

I see it, when I try to add a traveller to my trip.

OK, so that's on the website. Are you using the correct pass cover number? That's a capital letter “i” and 8 digits.

Also note that the Interrail website isn't the best place to book reservations. In fact, they can all be booked elsewhere, often cheaper. See this overview:

https://www.seat61.com/interrail-and-eurail-reservations.htm

yes

Yes it was the right number. The support now found a solution. Thank you aswell for the reservation idea!