When you attempt to purchase an app, the App Store may balk and tell you that "." If this happens, the solution is typically a simple one: Quit the App Store and wait a few seconds. Then relaunch the App Store and try again.

More generally, , just try again--clicking the Buy, Install (or Resume) button as relevant. If you cannot find a previously purchased app in the Purchases list, and you are correctly signed in to your account, press Command-R to refresh the page.

After selecting Sign In from the Mac App Store's Store menu, the . Or, the sheet may appear but any attempt to enter text in the Apple ID or Password fields will not work. The likely culprit here is third-party system enhancement software. Apple specifically cites Unsanity's and (Application Enhancer) as causing this. For what it's worth, I have APE installed on my drive and do not have this problem. However, I have seen confirmations of this symptom for FruitMenu.

The solution? Disable the third-party software. Hopefully, a future update to the software will resolve the conflict.

After clicking the Updates icon at the top of the App Store window, the (or, as Apple puts it, "become unresponsive"). This is a rare symptom, triggered by "certain third-party Mac NTFS software and/or connected partitions with versions of Microsoft Windows that contain the NTFS filesystem."