For a man whose flagship hedge fund was down more than 50 percent at one point last year, Michael Hintze sounds surprisingly upbeat. On a chilly winter afternoon, Hintze, 67, phones from London to talk about his firm’s reckoning this past spring, a time when the flagship fund’s structured credit instruments went terribly wrong and the firm’s positioning amid market turmoil sent that and one other fund plummeting to their worst-ever losses — wiping out billions of assets in mere weeks. It was an exceedingly rare stumble for CQS, a London-based hedge fund that had reliably minted money for investors more or less uninterrupted since its founding in 1999. Its flagship Directional Opportunities Fund had averaged gains of nearly 14 percent per year since its inception in 2005.