SJ (Swedish Railway) will release the reservations after 10th of December on the 9th of November. See below for instructions for how to make reservations at sj.se

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index2.html?postid=20744#post20744

What route do you plan to take to/from Abisko?

The trains between Stockholm and Abisko are run by VY, and not by SJ, and those trains are already bookable at sj.se 

If you plan to take the night train to Abisko my suggestion is that you make that reservation as soon as possible.

Thanks to all for your infomation. I am planning to travel from Copenhagen to Abisko via Stockholm using a Eurorail pass. Is it better to reserve the tickets through eurail.com or book directly from the local train company?

The latter. It's often cheaper and there seem to be fewer technical problems.

These trains are bookable only to 15.01. From 16.01 every day is written “sold out”. Will tickets for dates after January 15th go on sale from November 9th? I need a ticket Stockholm-Kiruna-Abisko for the middle of February.

Since when did tickets become so expensive? In all reports from recent years, inkl. 2021, people who have traveled on this train say that the tickets were relatively cheap 30-50 euros. Now I don't see a date with a lower price than 85 euros for a berth in couchette.

Deutsche Bahn has already released the timetable until February. But the direct Hamburg Altona-Stockholm SJ night train is missing there. Will this train run after the 10th of December?

It sounds like you're indeed looking at normal tickets and not at pass reservations (your pass is your ticket). Did you indicate you have an Interrail pass?

It will. I suspect reservation will also open on 9 November.

The price for a berth in a chouchette is 240 SEK.

Thank you.

I find it strange why Stockholm-Narvik bookings are cheaper than shorter distance bookings in Sweden? Is there a problem if I book Stockholm-Narvik but get off in Kiruna because it's cheaper?

I get the same price to Kiruna as to Narvik if you choose the direct train.

This train is the direct train. I see 240 sek to Kiruna, and 227 sek to Narvik via the browser, for the same train, I don't have the app.

I will travel a few days after Kiruna to Abisko, and then to Narvik. Is the booking only valid for one trip, or if I have a Stockholm-Narvik booking, can I use it next day from Kiruna-Abisko, next day Abisko-Narvik?

You need one reservation per journey so you need a reservation from Stockholm to Kiruna, then one from Kiruna to Abisko and a third from Abisko to Narvik. The 2 latter should be 50 SEK each  for a seat.

Reservations must be done at sj.se and not in the SJ app.

Well, I do see the same differences on the website:

Stockholm-Narvik is 48 SEK for a seat, 227 SEK for a couchette and 470 SEK for a T3 bed.

Stockholm-Kiruna is 50 SEK for a seat, 240 SEK for a couchette and 444 SEK for a T3 bed.

Both for the same direct train. I'd also be tempted to book to Narvik if I wanted to go to Kiruna. Nobody will stop you from doing that.