Hi Modestus,
I just had a similiar problem with the plasterboard coming away. I'm no expert on this, but you start by cutting the plasterboard around the boarder of the tiles to seperate the plasterboard under the tiles and the plasterboard which is exposed. Take the tiles out; put in a lump of plasterboard in the hole and retile over it... that's the way my tiler did it. Plasterboard can be cut to size easily, and you can throw up the tiles over this.
Although all did not go perfectly (see my post from today).
Darragh.