It looks like your belt has been tracking on the outer edge of the rear sprocket, possibly quite hard against it, hence the damage. You need to align the the rear axle to make the belt run evenly on the rear sprocket, this should be when bevel box is square in the chassis and the rear wheel is tracked straight. The guides on the sprockets should be guides for the belt, not restraints. Turning the adjusters for the axle evenly on each side is not a way to track the rear wheel, it can be way off like that.
