Mac App Store Rejection - App sandbox not enabled
Okay, solved this one myself and posting here for completeness. It would seem that despite the existence of the entitlement file in the project, unless the app is codesigned it will not recognise the sand box entitlement. This is even though the app validates successfully.
I had the same error message, but for me the problem was the fact that I forgot to sign the app.
I tracked down the problem to the Build Settings, Signing, CODE_SIGN_ENTITLEMENTS had multiple values. The values looked all the same, so I just re-entered the path for the entitlement file, the multiple values went away, and I no longer got the Sandbox not enabled error.