That's seriously bad news, Mario.
I don't think that a hairline crack in a rocker would significantly affect belt tension, since the pulley don't move in relation to each other. Only the belt tensioner and the belt itself affect its tension. Either it was too loose to begin with or the tensioner pulley has moved somehow.
Most likely scenario is that the cam pullet jumped a few teeth, piston kisses open valve. Even so, it's not usually the rocker that breaks.
Hope the damage is not any more serious than you already know.