ᐅ Interior wall wallpaper is coming loose—what can be done?

Hello,

I bought a house built in 1970 and originally planned to wallpaper the bedroom. Then I noticed that this wall is very unstable and is actually breaking apart, see photos:

On the outside, there are wood boards:

Now the entire interior wall is gone. What should I do next? Install drywall panels (plasterboards) and then plaster and wallpaper? Or plaster directly onto the surface?

A professional should take a look to determine the cause. It could possibly be a fungus. This might also be damaging the structure elsewhere.
It might be harmless, but as the owner, I would prefer to be on the safe side.

To me, this looks like Heraklit. These Heraklit panels must be attached to something, probably a single-layer masonry wall. On the outside, there is then a curtain wall facade made of the shown "boards."

I also have such or even worse "fun walls" in my old house. There, the wallpaper actually holds the plaster 😡
I have installed drywall partitions in front of almost all walls. Earlier, these were made of wood (battens + chipboard). The most recent walls use the currently common drywall method: framing + OSB + gypsum board. Then you can apply wallpaper of your choice on top.

ypg schrieb:

A specialist should take a look to identify the cause. It could possibly be fungal decay. That might also be affecting the structure elsewhere.
Maybe it’s harmless. But as the owner, I would want to be on the safe side.

There is no mold.
On the outside, there are boards. This is timber framing, which explains why it was loose.
But what is the best way to repair it now? Simply plaster over it or install gypsum boards?

Once the Heraklith boards are fixed in place, plaster them with gypsum plaster including a fabric reinforcement layer. Apparently, cement or lime-cement mortar was applied, followed by just mesh tape at the joints. This method does not work. We have had good experiences using lightweight gypsum plaster for this.

jcan schrieb:

If the Heraklith boards are solid, plaster them with gypsum plaster and reinforcing mesh. Apparently, cement or lime-cement mortar was applied, and only reinforcing mesh was used at the joints. That doesn’t work like that. We have had good experience with lightweight gypsum plaster.

Ah, okay. Very good. Yes, the board is very solid. There’s no vibration at all.
Would it actually be possible to improve the insulation value? For example, could I add another Heraklith board on top and then plaster it? There is space available.
Or could this cause problems, such as mold?