I’ve never watched a deck from this angle before so I could be way off, but… The belt looks too long, the far idler bearing may be bad, the idler arm is either bent downwards or the deck where it’s attached is bent, and the brake arm disengages too far way from the spindle it brakes, as though its fastening is loose or the fastener hole has ovalled out, so the brake is actually touching the belt when disengaged. Did I miss anything?😉🔧👍 Or, nothing! 😉