Have these marks appeared since the damp proofing work was carried out.
To my knowledge all internal rendering would have been removed to a height of 1 metre ( for rising damp or the whole area for penetrating damp) in affected area for treatment, so why have you got wallpaper there, unless you have papered it since the treatment. You should have been advised not to wallpaper for six months, by which time the wall should have dried out.
I was in the damp proofing trade only a short while ago, things could have changed in the last few years but I doubt it.