Complex sentence: Levine's Boston Symphony contract limited his guest appearances with American orchestras but Levine has conducted regularly in Europe, with the Vienna Philharmonic, Berlin Philharmonic, and at the Bayreuth Festival.
Split into simpler sentences: Levine's Boston Symphony contract limits his guest appearances with American orchestras. However, Levine has conducted regularly in Europe, with the Vienna Philharmonic, Berlin Philharmonic, and at the Bayreuth Festival.