Jewish scapegoating

Reading an otherwise excellent book: Monasticism, Gateway to the Middle Ages (Duckett, 1938), I am disturbed by its casual acceptance of the claim that the Jews in a certain medieval French city, Arles, attempted to betray the place to barbarian (Frankish) besiegers by tossing down a message stone from the walls -- a claim that came just in time to take the pressure off the city's ecclesiastical leader, one Caesarius. I searched through James Carroll's Constantine's Sword and found no mention of the episode. Rather surprising, since Carroll's theme is the tortured relationship of Christianity and Judaism, and Caesarius was the controversial Vicar Apostolic of all Gaul and Spain at the time. His allegiance to Arles was several times questioned, since he was of Burgundian birth. What would the local Jews possibly have gained from having Arles pass into Frankish hands? Even the best books of history are written by mortal men and women, and a good scholar doesn't unquestioningly swallow their every word, however authoritative.