How do I repair the corner in the best way

[Weihrauch]

Hello!

I'm new to the forum so I apologize if I'm posting in the wrong section.
We have a house from the late 60s and there is an inner corner, against the outer walls, where there is an old crack.
I have taken down a textured wallpaper and started to remove the last wallpaper before what I believe is aerated concrete.
How do I tackle the crack in the best way and how do I repair it?

 

[Jansson69]

You can knock down anything loose in the crack and vacuum it, then repair it with, for example, Lagavägg from Finja. Once you have repaired it smoothly and puttyed and treated the wall in preparation for painting or wallpapering, you apply adhesive sealant in the corner before the final surface.