With supercharger belts with a manual tensioner that comes stock should be "broken in" Belts stretch and they need to be allowed to stretch without breaking things. Did you do a belt break in period or did you just crank down on that JR tensioner?
Shaft breakage is pretty common with overtightened belts....This is why I suggested using the stock autotensioner...gotta get a new snout for you present charger and swap to the auto tensioner
Ok I reread and you were on the stock tensioner...What sized belt were you running?