Problem: The Mac App Store on my laptop refused to allow me to update existing applications, download new ones, sign in to my account or view existing downloads. Trying the “Store > Sign In” and “Sign In” link from the Featured page both refused to display the usual login dialog. Attempting to update existing applications showed the usual “spinner” in the top toolbar with no progress.
Dead ends: Suggested on the Apple Discussion forums, there were several items suggesting anti-virus and firewall involvement. None of these were applicable to my situation and I was attempting all of these actions from an unrestricted TekSavvy cable connection.
Solution: This post on the Apple Discussion forums provided the initial help, but was incomplete in its solution. First, close out the App Store, then enable the debug menu by running
defaults write com.apple.appstore ShowDebugMenu -bool true
from the Terminal. Launch the App Store again, and choose Debug > Clear Cookies and Debug > Reset Application. Quit and relaunch the App Store, and you should be able to sign in and download updates successfully.