User named [Update Needed] after installing yosemite with encrypted drive

• Log in into the Mac as administrator.
• Backup the Mac with Time Machine (optional but recommended)
• Launch System Preferences selecting System Preferences from the Apple menu.
• Click the Security & Privacy preference pane.
• Click the FileVault tab.
• Unlock the pane clicking on the padlock (administrator password required)
• Disable FileVault and wait (…about 3hrs with a 250GB no SSD HD)
• Restart the Mac (optional but recommended)
• Go back the FileVault tab.
• Enable FileVault and wait (…about 7hrs for me!!!)
• Restart the Mac.
  The unknown user account will disappear.
  This should also now allow you to set automatic login.

Sources: FileVault and update needed at the login screen: How to solve
Fresh install of Yosemite adds [Update Needed] user
Login screen shows "update needed" but no updates are available