Answer: rub the paper with a sponge moistened with hot water (or even dish soap), or scrape the paper with a spatula.
When setting the wallpaper, it is possible that glue stains are deposited on the paper inadvertently. First hypothesis, the glue is fresh. Act immediately before it is dry, take a sponge, moisten it with hot water and gently rub the wallpaper until the stain is gone. Second hypothesis, the glue has already had time to dry. In this case, lightly scrape the surface concerned using a spatula.If it does not work, make a mixture of hot water and detergent, moisten a sponge with and rub very gently so as not to damage the wallpaper. Above all, it goes without saying, do not use the scraping side of the sponge! Normally, the glue should go away without problem.