[Osem]

I'm no builder but to me that looks more like a typical condensation problem and if the other factors I mentioned apply then I'd probably bet on it. The brickwork outside doesn't look damp/green and I reckon if significant water was getting inside from the roof, the damage in your cupboard would be far worse. The flashing under the gutter looks a bit poor and there is a gap in the woodwork but all that is under the soffit so unless there's a leak from the gutter/roof above it I don't see much water getting in that way.

Presuming your room is on an upper floor, any idea what's directly below your cupboard and whether there's a similar damp problem there? If there isn't it adds weight to the condensation theory I'd have thought.