Software Update: Could not be Verified

Please read this whole message before doing anything.

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

The purpose of this exercise is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login. Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode and log in to the account with the problem. The instructions provided by Apple are as follows:

• Be sure your Mac is shut down.
• Press the power button.
• Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
• Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).

Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.

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(s)?

After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of the test.

Select  ▹ System Preferences ▹ Network ▹ Advanced ▹ Proxies. If any proxies are selected, deselect them, apply your changes, and try again. You must apply the changes before they take effect. You don’t need to disable passive FTP mode.

Check that you have enough free disk space to store the updates. Almost certainly, you do. You can also try reinstalling the Mac OS, but that probably won't help either. Some people have suggested emptying the folder /Library/Updates, but I have no reason to think that helps.

It's a network issue. I've seen reports that some university networks, in particular, block Software Update. Go to an Apple Store to download the updates, if possible.

It's still a network issue. Try rebooting your router.

Go to Disk Utility, choose your Macintosh HD and press Repair Disk Permissions. That worked for me, at least.

@ttretjak: Thank you very much. That solved my problem as well! DISK UTILITY - repair disk permissions.