Any reputable plasterer knows you should never skim on to paper as this causes a multitude of problems , not necessarily has it caused the damp but it will have played its part in showing up the damp. It is a big no no in my eyes!

The wall paper will not be the cause of the dampness, it is however poor workmanship plastering over wall paper. Due to the moisture in the wet plaster, when initially skimming, this may cause the wall paper to lift creating an air bubble between the wall and the paper. When the plaster sets it is only adhered to the wall paper and not the wall therefore creating a weak spot which may lead to cracking or plaster falling off. The answer to the damp issue could be a multitude of issues for example poor ventilation a breach in the dpc or bridge in your cavity wall allowing moisture to pass through.