Rear diff pulley wrecked after 1st run on Mi4LP

[stnorm]

Ive just built my Mi4lp and was really looking forward to seeing how she went. Alas after a couple of mins it felt like i had no rear wheel drive and after inspection the belt had melted both fences off the diff pulley and fell off. New diff pulley and fences ordered but any ideas why this might have happened. im using a Norsram rure 10.5t motor with a 21t pinion onto the stock spur gear powered by bionic's 4000mah, 40c lipos. Im new to belt drive cars so could it be the belt tension that is at fault? The rear belt has about 3mm up/down play in it. Should I take the shims out of the diff pulley to allow more free play? Any advice would be much appreaciated