Can't download anything from AppStore

Please read this whole message before doing anything.

This procedure is a diagnostic test. It won’t solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of this test is to determine whether the problem is localized to your user account. Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”

While logged in as Guest, you won’t have access to any of your documents or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this behavior; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault in OS X 10.7 or later, then you can’t enable the Guest account. The "Guest User" login created by "Find My Mac" is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

a problem with my Appstore account

I doubt it, but if you have another Mac, you can test the theory. Otherwise, see below.

If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.10 ("Yosemite"), but the procedure is the same for OS X 10.7 ("Lion") and later.

I'm just going to jump in here to say that I have the same problem, so if getting certain data from a second computer would be useful, here I am.

I recently went directly from Snow Leopard to Yosemite 10.10.4 - and any apps from the App Store that needed updating thereafter worked out fine. When 10.10.5 came out, I had no trouble downloading and installing it. But since then, two updates have given me the "error" message Dave mentions. Since I upgraded the OS by a lot, I've upgraded/updated a lot of apps, so it may be that my particular situation is confused as all ****, but as I say, if I can be of use, just ask.

Hey guys, especially Dave -

I, too, reinstalled my system. I, too, could download the balky apps afterward. But now, two weeks later, I get the error message again. I'm not about to reinstall the system again, so I'll live with getting my updates from a Genius…but I wonder if Dave can still download things from the App Store.

PS. I, too, use iTunes 10. Could the reinstallation of that after the system reinstall be the cause? If so, I still prefer iTunes 10 to easy App Store downloads, but if that leads Linc in a direction I'd be happy to hear it.

Thanks guys.