Anyone who’s ever tried to mount a play knows that screw-ups, accidents and misfortunes tend to mount as opening night approaches. But Sinnerman Ensemble’s current production of Anton Chekhov’s Ivanov seemed especially cursed just before its Thursday October 1 opening. In light of Sinnerman’s backstage story, supplied by the company’s executive director, Calliope Porter, it’s amazing that the tricky, two-and-a-half-hour show turned out to be one of the best of the year. Porter’s litany: