Complex sentence: At the start of Blum and Massine's company, Massine ran into trouble with Col. de Basil: the ballets which Massine choreographed while under contract with Col. de Basil were owned by his company.
Split into simpler sentences: At the start of Blum and Massine's company, Massine ran into trouble with Col. de Basil. Massine realized that the ballets which he choreographed while under contract with Col. de Basil were owned by his company.