NOT=nicht to single, or perhaps even 1st, as we read some weeks ago that the comp running this trains does not allow cheapo (geizbillige) passholders to use such expensive compartments for so little!

Hmm, not nice. Nor can I book individual seats on the vy.se pages without buying a train ticket.

You can get a 2nd class sleeper on those night trains. Maybe they were sold out and only the 6-person couchettes were available. 

What dates do you want to travel?

That's true, but on the Interrail website I was only able to reserve the 6-person sleeper car, although vy.se also offers the 2nd class sleeper compartments (unfortunately I didn't see that until later). Actually, I just want to exchange the Interrail reservation for a sleeper compartment. Let's see, I wrote to vy.se.

Ok. I didn't realise that you had made the booking already.

Tickets for trains in Sweden are best booked at sj.se

Enter "Interrail or Eurailpass" as "SJ Prio /Period ticket" and an I or E followed by any 8 didgits as card number.