This procedure will delete certain temporary and cache files used by the software update process. The files are automatically generated and don't contain any of your data. Occasionally they can become corrupt and cause problems such as yours.
Back up all data and read this whole message before doing anything.
Triple-click anywhere in the line below on this page to select it:
/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with a subfolder selected. The subfolder has a long name beginning with "zyx" and ending with "68". Move that subfolder to the Trash. Don't move any of the other subfolders with similar names. You may be prompted for your administrator login password. Restart the computer and empty the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted because a line break is included. Press return.