Symptom
When you open Backup, an alert box appears with a message that says the application could not connect to .Mac to verify your .Mac membership.
Solution
Move the computer you want to back up to a place where proxy severs are not required. Backup 1.0 and later cannot be used with proxy servers.
The first time you open Backup your .Mac account is verified, which is why the computer needs to be located outside of proxy servers. After your membership has been verified, you can change the backup destination to CD/DVD. The next time you open Backup, the authentication process is not required. You don't even need to be online to back up to CD/DVD.
Note: When opening Backup after the first authentication, you will see an alert box with this message: "iDisk Server is Unreachable. The iDisk server is either unavailable or unreachable. Please try again later." The options are to select "Open Network Prefs" or "OK". If you click "OK", Backup will open with CD/DVD selected as the backup location.