1. Make sure you're still signed in to the App Store. If you are, sign out and sign back in.
2. Take the steps recommended in this support article.
3. If there's no change, continue as follows.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
~/Library/Cookies/com.apple.appstore.cookies
Right-click or control-click the line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item selected. Quit the application if it's running. Move the selected item to the Trash. Relaunch the application and test.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.