Launch Disk Utility and run Repair Permissions on the startup volume. Try whatever you were doing again. If there's no change, continue as follows.
Triple-click the line below to select it:
/private/tmp
Right-click or
control-click the highlighted line and select
Services
▹
Show Info
from the contextual menu.* An Info dialog should open.
- Does the dialog show "You can read and write" in the Sharing & Permissions section?
- In the General section, is the box labeled Locked checked?
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). Open a TextEdit window and paste into it (command-V). Select the line you just pasted and continue as above.