A modal dialog box is showing in the QuickBooks user
Close all dialogs
Problem
You have a modal dialog box open in QuickBooks:

Description
In Connex, you run the sync and get this error:
A modal dialog box is showing in the QuickBooks user interface. Your application cannot access QuickBooks until the user dismisses the dialog box.
You cannot sync, while a modal box is open.
Solution
In QuickBooks, close any open windows that ask for OK: