I don't know if it helps, but I was getting the same error in this scenario:

1. I created a new Apple ID account. I signed with this account on my Mac, everything worked - App Store etc.

2. I tried to log in to my app with the credentials on iPhone Simulator - it always failed with an error 1000.

3. Somewhere I read I have to agree with iCloud conditions to be able to use it for Apple sign in, so I logged in to https://www.icloud.com/, agreed the conditions and then I was able to sign in.

Maybe it helps someone.