The invoices don`t export due to this message: There was an error when converting the quantity 1 in the field item quantity.

QuickBooks error message: You cannot set the quantity for items of this type.

Please make sure the Products exist in QuickBooks as Inventory Items. The Adjustment Item should exist in QuickBooks as an "Other Charge" item. If any of these have been changed in QuickBooks to other item types, then this error would occur.