We can only think about a solution if you provide specifics: route, date and departure time.

attached screenshot of my route

You can get the first 3 on entur.no : add Interrail/Eurail and deselect adult. A seat should cost 50 NOK and a reclining seat 600 NOK. Other accommodations seem to be sold out.

There is a warning on the website that the Oslo - Bergen night trains (and back) are replaced by bus but I'm not sure if it applies to the whole journey or only to some part.

Book from Sundsvall on sj.se : add Interrail as a discount and type any pass number. 65 SEK for the first train and 62 SEK for the second ond.

Definitely book ASAP.

Thanks a lot. I have done exactly that and it works, although I don't know exactly the oslo-bergen bus warning either.

Does sj.se work for you? It hasn't worked for me since last week :/

Yes it works. I'm in the Netherlands right now so I'm not sure the country is the issue...

Pity you travel by night across most of Norway, you’re gonna miss al the good views! Truly some of the most impressive routes in Europe.

Sj.se doesn’t seem to work for me too…

I changed the VPN to a Norwegian or Swedish one using a free VPN app. And it really works

It still doesn't work for me, with or without VPN (NordVPN).

I'll continue to bother SJ about it.

Which one do you use? I didn't have any luck with mine :(

Really weird. I'm in Amsterdam right now and I can use the website as usual (Chrome on my Samsung).

It's loading right now (and did yesterday evening, too) but didn't work most of the week (and even didn't work 15 minutes ago). Maybe try again right now. 

Indeed, it works again. For now...