One of the pulleys is out of alignment. The shredding is caused by one of the pulleys with grooves and sides but the cause can be a tipped or turned grooved pulley OR a smooth one. Sight down from on top. You should not see the belt peeking out on any lower pulleys. If you do, the lower one or the one right above it is out of alignment. Typically that is caused by worn bearings and occurs most commonly to the smooth idler pulleys.