The check valve then ignited and ended in the explosion that destroyed the spacecraft.
The reason for the leak itself seems to be the explosive reaction between titanium and NTO at high pressure, a thing SpaceX says it was unaware of, as this material “has been used safely over many decades and on many spacecraft from all around the world.”
Armed with this new knowledge “will lead to further improvements in the safety and reliability of SpaceX’s flight vehicles.”
To fix the problem in the rest of the Crew Dragons, SpaceX says it has already begun taking additional steps, including the elimination of any flow path that could allow liquid propellant to enter the gaseous pressurization system and the replacement of the check valves with burst disks.
The explosion of the Crew Dragon caused some changes in the flight program of the company. Another Crew Dragon, originally assigned to the second demonstration mission to the ISS will now be used to perform the first in-flight abort test, and the one assigned to carry humans for the first time in space has been designated to perform the second uncrewed flight.
You can have a look at SpaceX’s findings about the April explosion, complete with a technical analysis, in the document attached below.