iTunes 9.0.3: Error 42408 or Error 42404 possible resolutions
When downloading purchases from the iTunes Store or authorizing those purchases on your machine, you may encounter either Error 42408 or Error 42404 stating that the action could not be completed.
To help workaround this issue users can:
Turn off automatic syncing
Disconnect your iPod or iPhone if it is connected. Navigate to iTunes Preferences > Devices and select the option to "Prevent iPods from syncing automatically." Connect your iPod or iPhone and test the issue.
Reset Safari's cache
Quit iTunes and open Safari. From Safari in the menu bar, choose Empty Cache. Quit Safari, open iTunes and attempt to recreate the issue.
Delete iTunes preference files
Navigate to (home folder > Library > Preferences > com.apple.itunes.xxx.plist) and move the file to the Desktop. Test the issue. If it is resolved, move that file to the Trash (a new iTunes preference file has been created in its place).
Attempt to authorize a new user
Quit iTunes and open System Preferences. Click Accounts and set up a new user. Be sure to include Admin privileges. Log in to your new user and open iTunes. Attempt to recreate the issue by authorizing the account, purchasing a new song, or syncing an iPod or iPhone. If the issue is resolved, check your original account to see if it is resolved there as well.
Note: These are workarounds for this issue and may return in the future. Be sure to update iTunes when new versions are released.