Complex sentence: It is unlikely that Minkus ever worked again for the Imperial Theatres in an official capacity due to differing accounts of his involvement in two important productions between his retirement in 1886 and his final departure from Russia in 1891.
Split into simpler sentences: It is unlikely that Minkus ever worked again for the Imperial Theatres in an official capacity. Differing accounts survive that tell of his involvement in two important productions between his retirement in 1886 and his final departure from Russia in 1891.