My first reaction is that the problem may lie with a strong, cold wind combined with poor, or no, insulation in the corners, thus causing condensation, or a combination of wind and rain leaking through the garage in the corners. If the damage is occurring from the outside, possibly your flashings are poorly placed and excess water is dripping down on the affected areas. If the problem is inside, it is most likely an insulation problem. Often, when a building is being constructed, the walls and attic are well insulated, but the corners are neglected. My guess is that the moisture in the corners is being caused by condensation from the inside. A quick fix to this problem is to install a piece of rigid foam insulation from ceiling to floor, 5 cm x 5 cm (2 in. x 2 in.), cut at 45°. Do this for all four corners. You will lose a small triangle of space in all four corners, but the added insulation should correct the problem.