A partial solution:
- Create a new user account on your Mac with the same Apple ID, and give it admin privileges.
- Log in as the new account, and run App Store.
- Updates of non-pre-installed apps (i.e. nearly all of them) should now work fine.
- Trying to accept pre-installed apps (e.g. iPhoto, iMovie, Garageband) still doesn't work; the spinner runs and nothing else happens.
I suspect that there's a bug in accepting pre-installed apps that clobbers the per-user App Store settings. Let me just work out which files they are...
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OK, these are the files:
~/Library/Caches/com.apple.appstore
~/Library/Caches/com.apple.SoftwareUpdate
~/Library/Caches/com.apple.storeagent
~/Library/Preferences/com.apple.appstore.plist
~/Library/Preferences/com.apple.SoftwareUpdate.plist
~/Library/Preferences/com.apple.storeagent.plist
~/Library/Cookies/com.apple.appstore.plist
Throw these in the trash (but don't Empty it) along with any files that have the same name with .lock at the end. Then try again. If that doesn't work, try the new account workaround I mentioned above.