App Store "An Unknown Error Has Occurred", Apple Support fix was only temporary. How fix permanently?
I found a solution. Another forum post describing similar issues suggested deleting /Library/Preferences/SystemConfiguration/NetworkInterfaces.plist. I backed that file up, deleted it, and restarted. I can now log in and out of AppStore, access all of my AppStore purchased apps, and it 'sticks' across shutdowns and restarts.

Process is pretty simple, I'll spell it out for future reference:


Open Terminal
cd /Library/Preferences/SystemConfiguration
cp NetworkInterfaces.plist ~/NetworkInterfaces.plist.bak - Make a hasty backup in your home directory
sudo rm NetworkInterfaces.plist delete the original
Restart your Mac
Open AppStore and try to sign in. It should work.


Apparently this works because the MAC address is used as part of the receipt/authorization process.