My hint for that problem.
Download the latest beta as ispw file.
I did it too. I had the same problem.
Don't do the official way by deleting public beta profile. If you do that, you have to install 9.2.1 first. Thats the problem, because at that moment you have to pair the phone with your ID.
Download the ispw of 9.3 b3 and make an install with iTunes. I did. After installing directly 9.3 it was possible to backup. In this case Apple ID and 9.3 b3 works together. Good luck
Download the latest beta as ispw file.
I did it too. I had the same problem.
Don't do the official way by deleting public beta profile. If you do that, you have to install 9.2.1 first. Thats the problem, because at that moment you have to pair the phone with your ID.
Download the ispw of 9.3 b3 and make an install with iTunes. I did. After installing directly 9.3 it was possible to backup. In this case Apple ID and 9.3 b3 works together. Good luck