QuickBooks: The user you are trying to log in with is already logged in from another machine.
The user that will be used by Sync & Save to log in to QuickBooks is configured in QuickBooks under Edit | Preferences | Integrated Applications | Company Preferences. Select Sync & Save in the list of Integrated Applications and click Preferences.
Make sure to assign Sync & Save a login user account that will not be used by other users.