The interior wall is peeling off when wallpapering, what can be done?

To be honest, I am cautious about making a clear statement, as it gets more complex with half-timbered constructions the more insulation you add on the inside. For one property, we did it like this, with double Heraklith. The panels were completely historically made, with staking and clay. Everything was rebuilt from the inside out.
BUT I do not want to apply this generally to your property. Since it apparently works with the one board, it should continue to work. But for more, you should ask someone who inspects the site and has experience with interior insulation in half-timbered buildings.

Whether there are no problems cannot be said for sure from here, but it is very unlikely since you are only replacing the plaster. For gypsum plaster, a good centimeter is sufficient. I would completely embed mesh and then not apply wallpaper anymore, but remain diffusion-open. So ideally a dispersion silicate paint, for example.

Ok thanks. I wanted to use Rotband plaster for the plaster now. Is that okay?