Complex sentence: It is sometimes stated that he was a son of André, castellan of Vitré and of Agnès de Mortain, but this appears to be based on a confusion between Vitré in Brittany and Vitry in Champagne.
Split into simpler sentences: It is sometimes stated that he was a son of André, castellan of Vitré and Agnès de Mortain. However, this appears to be based on a confusion between Vitré, in Bretagne, and Vitry, in Champagne.