Complex sentence: The problem was traced later to a faulty mechanism on the Soyuz; this was not discovered until after the Soyuz 26 flight performed an EVA to inspect the second docking port on Salyut 6.
Split into simpler sentences: The problem was traced later to a faulty mechanism on the Soyuz. It was not discovered until after the Soyuz 26 flight, however, which performed an EVA to inspect the second docking port on Salyut 6.