For anybody encountering this in the future, it turned out that the .mov being in a different folder than the project (Downloads vs Documents) was causing Xcode to silently fail. I had never rejected permissions for Xcode to access a folder. Temporary solution: Putting the .mov into a public folder such as "/Users/Shared/..." and re-adding it to the scheme seems to have fixed the issue. This doesn't explain why it worked once but not in subsequent runs.