Airbus has traced the problem to the 7449 aluminium used in the wing ribs. 7449 is more sensitive to the way the parts are assembled on the wing. They ruled out flight loads or fatigue as causes.
My guess is that 7449 is lighter than the other stuff. What a disaster, it looks like the ribs and the feet are one single milled piece (pos). Not much of a sheetmetal guy but this does not seem like a simple fix.