Complex sentence: The only failed mission of the Falcon 9 v1.1 was SpaceX CRS-7, which was lost during its first stage operation, due to an overpressure event in the second stage oxygen tank.
Split into simpler sentences: The only failed launch of the Falcon 9 v1.1 is SpaceX CRS-7, which was lost during its first stage operation. This was due to an overpressure event in the second stage oxygen tank.