When doing a clean re-installation of El Capitan (highest supported version) from a bootable USB on an older Mac (MBP, 15-inch, Late 2008), I get this error:
An unexpected error occured while signing in.
The error shows up, while logging in with the Apple ID into the App Store in the installer.
I tried with different Apple IDs,
Also tried running these commands in the Recovery Terminal and restarting: