Complex sentence: The problem was traced later to a faulty mechanism on the Soyuz but this was not discovered until after the Soyuz 26 flight, who 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.