If X-Plane.app is installed by any method other than via the Laminar Research installer, it can be flagged for quarantine by OS X. This can happen if the app is moved via USB, or downloaded from the internet, such as from cloud storage. Being quarantined can in turn lead to the app being sandboxed.
To remove the quarantine marking:
- Open Terminal.app
- Type xattr
- Type a space then -cr
- Type a space, then drag X-Plane.app to the terminal window.
- Press return.
This should remove the quarantine flag so that X-Plane can run again.