Replacing the Boston Symphony Orchestra’s music director James Levine, who was suffering from persistent sciatica, Christoph von Dohnányi took over two performances of Strauss’s unclassifiably hybrid entertainment Ariadne auf Naxos – ‘a very serious trifle’ according to its librettist Hugo von Hoffmanstal – performed at the orchestra’s summer home of Tanglewood but with the students of the summer school, in a production by Ira Siff, best known internationally as co-commentator on Metropolitan Opera broadcasts.