We could not complete your update. "A server with the specified hostname could not be found".

Thanks this worked!

For the people who don't know how to do this:

1. Open System Preferences
2. Click on Network
3. Click on the lock to 'open it', in the lower left corner of the window. You have to enter your password.
4. In the left menu select the active connection type
5. Click on the Advanced... button
6. Open the DNS tab
7. Select the DNS server that is listed
8. Change the DNS server by using the + button
9. Type 8.8.8.8
10. Click on OK and then on Apply
11. First quit and then launch the App Store again, and it should work!

Triple-click the word "telnet" in the line below on this page to select the whole line:

telnet swscan.apple.com 443

Copy the selected text to the Clipboard by pressing the key combination command-C.

Launch the built-in Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

Paste into the Terminal window by pressing command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.

You should almost instantly get three lines of output below what you entered, the last of which is this:

Escape character is '^]'.

If there's no output within ten seconds, consider that a failure. Otherwise, please post the output in a reply. You can then quit Terminal.

Please read this whole message before doing anything.

This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.

Step 1

The purpose of this step 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, 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.

Step 2

The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Please take this step regardless of the results of Step 1.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of Steps 1 and 2.

I don't know what happened to the original poster, but I have this problem on my Mac Book Air running latest update of Yosemite 10.10.5.

I have performed the above steps, and in all cases telnet just hangs after the Escape character line (including in Safe Mode).

What other steps can I take to diagnose the problem?

Update: A few minutes after posting this (after restarting and testing the Telnet step one final time), I tried the App Store update again, and this time instead of an error message it just hung, with a spinning gear and a grayed-out Update button. Then, about 10 minutes later, I toggled tabs (-> Featured -> Updates) and found that it actually has a progress bar and had been downloading the update during those 10 minutes! I guess it's fixed.

My guess, a server restart/blip/connectivity issue that coincidentally was resolved after my post. Best of luck, if anyone else runs into this!

I solved this problem by changing DNS to 8.8.8.8. Don't ask my why this worked 😕

Thanx for the tip JWilson@ch!

I only logout and login back from my iCloud account. System Preferences > iCloud. May be the simplest way. Good luck!