I had more than enough to do in the rest of the house. I didn't want to take this on right now! The damp could stay for a bit to be solved later. The house won't fall down that quickly. But the wall looked awful now. And I went for the student solution: just buy a big piece of cloth to hide it. And so I did! It makes my room look indeed like a student room but so be it. It's OK for now! And I moved my dehumidifier from the kitchen tot he bedroom and it's making a big difference. I'll get to the core of this issue when the ground floor is done!
What it looked like with the wallpaper having come down
What it looks like now