From http://www.tonymacx86.com/el-capitan-desktop-support/173653-el-cap-app-store-failing-login.html
Steps to fix for my issue:
- delete all interfaces in the Network Pane of the Control Panel
- delete Networkinterfaces.plist from /Library/preferences/systemconfiguration
- restart
- add Ethernet interface in the Network control panel pane
now my ethernet is registered as en0. App store is working. iCloud seems to be connecting. iMessage is logging in .. but not really working (i see activity, but messages won't actually receive or send...)