I keep getting an "unknown error" in App Store

Question

Help please! when I try to purchase an App from the store it gives me an "unknown error" and won't let me purchase anything. can anyone help?

Answer 1

Here is a step-by-step of what worked for me:
Create a new user account - let's name it "Test" - I did not make this an administrator account
Log into that new user account (skipping the AppleID/iCloud screen)
Select "Software Update…" from the Apple menu - you will be asked to enter an administrator name and password in or to check for system updates - just provide your normal (admin-enabled) user name and password
If you have any updates available, click "Update All" and log into the AppStore with your normal AppleID/password (this should work since you are in a new user account)
(If you don't have any updates available, I assume that you can also log in using the menu item in the Store menu of the AppStore application though I have not tried that)
As soon as the download starts, you can quite the AppStore application
Log out of your test account and back into your normal account
Open /Applications/Utilies/Terminal
Type the following command:
sudo cp  /Users/test/Library/Cookies/com.apple.appstore.plist Library/Cookies/
Open the AppStore again and you should be able to log back in
What does this do? It seems that the com.apple.appstore.plist file in your normal user account has become corrupted - we use a new account and create a new one (which is interchangebale since we use the same AppleID) and move that one back to the main account with the corrupt file.
Andreas