The piece was moving along when everything suddenly collapsed and music director Franz Welser-MÖst uttered, “Sorry.” He took the ensemble back a bunch of bars and tried again, to no better effect. “You see how difficult this is,” the conductor said to the audience. “It worked this morning.” Finally, Welser-MÖst got his meters straight, and the music proceeded to its inevitable, touching end. One suspects the weekend’s remaining performances will go more smoothly.