While I am not too familiar with the history of 1960's building materials in Denmark, it looks like vertically installed early drywall which may or may not have a coating of plaster applied.
If it feels like drywall under the plaster than it is drywall.
The plaster can be floated without the need for the fiberglass unless you have severe cracking, which I don't see from here.
Maybe this will help you identifying what you have: remove the cover plate from an electrical outlet on the wall and examine the cross cuts around it. Then report back.
I think you have something that is called autoclaved concrete. Its like concrete that has been pressurized with a gas to make turn into a foam like material. It is fairly lightweight and is glued or mortared together. It is usually stuccoed on the outside and may be stuccoed or plastered on the inside. It does not have the R-factor of a plastic foam such as styrofoam, an 8" block is about R-11. But it is a continuous insulation so it works pretty good.
If the house is two stories, the blocks should be at least 11" thick and double 8" blocks used for three story. I saw a lot of that type construction when I visited Germany.
I don't know why you can't paint it like it is unless you just don't like the texture. You will need to wash off any residual glue from the wall paper first. If the bare blocks have not been covered with plaster, you could put another coat on. You can sand the old plaster but I would not sand the bare blocks. Back in the late 50's, textured plaster was semi popular so you might have this.
For wall coverings check on these products.
Don't know if they are available in Denmark ?
Plaster In A Roll.
Clay Roll-On Plaster.
I have also used plaster bandage ( Plaster of Paris in a roll ) to cover cracks.
Also look into a veneer plaster.
Yes, this is a construction type which is common in Germany. The interior walls are light weight cinder block. Electirical is fastened directly to the surface and then the entire wall is plastered over. Most Germans would then put "Raufasur" on the walls. This is probably what you had and why it stripped so easily. It is a heavy reinforced paper which has a heavy texture on the surface. It can be painted or left with the natural white color. Your easiest course would be to simply replace the old paper with new. Knock down any high spots with sandpaper or a sc****r and simply hang right over the old paste, assumning there is no mold or mildew present. Otherwise, remove the mildew first.
After hanging, it is your choice as to whether to paint it or not. Raufasur is normally only sold in white. It does come in dozens of textures and designs, anywhere from stucco like to patterned tin ceiling looks. Sorry, I have no idea or what the raufasur is called in Denmark, but I know that it is used there too.