Andreas Conrad (Mime) and Alan Held (Wanderer, seated) in “Siegfried” at the Washington National Opera, directed by Francesca Zambello (photo by Karin Cooper) Washington National Opera continued its epic American Ring Cycle on Saturday night, opening a production of Wagner’s Siegfried that was plagued by vocal and technical troubles but was still an evening of revelations. Director Francesca Zambello continued the work begun in her 2006 Das Rheingold and 2007 Die Walküre, which transferred…