QuickBooks Desktop sync error: A modal dialog box is showing in the QuickBooks user interface or Unknown error (errorCode: MessageModalDialogOpen)
Why the sync error occurs
This error occurs when you are attempting to setup your sync, or run a sync, but an open window or modal in QuickBooks Desktop is waiting for a user response.
How to fix the sync error
To resolve this issue, return to QuickBooks Desktop and close any dialog boxes.
If you don't see any open dialog boxes, then close out of all your running programs (QuickBooks, 3rd party programs, internet browser, etc.) and try again.
If the error continues, reboot your computer and sync again.