As from today it is possible to make Interrail/Eurail reservations for both seats, beds in sleeper and chouchette on the SJ Euronight between Stockholm and Hamburg Altona. For dates after the 10th of December, reservations can only be made at sj.se .
According to the information I have reservations will not be possible through ÖBB as there are problems with the communication between the ÖBB and SJ reservation systems.