When the Mac App Store first came out, a number of users found they were not able to log in to the system. While sometimes initial configuration bugs can prevent these normal behaviors from occurring, it was soon found that people who had the system add-on "FruitMenu" by developer Unsanity installed seemed to be more affected by these bugs.
Weon this issue when it first cropped up, and more recently covered some why people might not be able to log in to the Mac App Store. Recently MacFixIt reader Brian wrote in with some information that the problem may be because of a bug with Unsanity's Application Enhancer framework.
Oddly this issue does not affect all Mac users, but many who are affected have come to rely on Unsanity's products for day-to-day work. If you are experiencing this issue and alternative fix attempts do not work, then you will have to uninstall the program in order to use the Mac App Store. To do this, go to the FruitMenu Web site and download and open the installer for it. Then click the "Uninstall" button to have the installer move the application's files to the trash. When this is done, log out and log back in to finish the uninstallation.
Unsanity did not respond to inquiries about this problem, but in a recent tweet, the company mentioned that the Mac App Store incompatibilities may be because of a small but significant typo in FruitMenu's code, and yesterday a new beta build of FruitMenu began testing. It appears a full fix is in progress and may be available in an upcoming FruitMenu update.