I have had the same problem with not being to log into App Store even though I have an active Apple ID. Here's how I fixed it:
1) Ran EFIstudio and extracted my ethernet device's EFI string.
2) Saved resulting XML file to a text-only file, and then clicked "write to com.apple.Boot.plist". Take note that this is the com.apple.Boot.plist that is in /Library/Preferences/SystemConfiguration, not the one in /Extra/. Put aside the ethernet XML file for later.
3) Since I use an EFI string to run my 9600 GT as well, I also extracted the EFI string into another XML file by using OSX86Tools.
3) Spliced both XML files, then imported the resulting file into OSX86Tools again to create the Hex code.
4) Edited com.apple.Boot.plist that is the /Extra/ folder, and under <key>device-properties</key>, I replaced my old videocard only Hex code with the combined video-card+ethernet Hex code.
5) Go to System Preferences/Network, and delete all the devices on the left pane.
6) Go to /Library/Preferences/SystemConfiguration/ and delete NetworkInterfaces.plist. Reboot.
7) Once rebooted, go back to System Preferences/Network, and start adding devices again. Make sure you add Ethernet first, followed by WiFi (if you use WiFi as well) and all the other devices in there. You should be able to get into the App Store.
Hope this helps.