Field Integrity Exception comes when you have space between your transaction line items.
If you have a large number of records in QuickBooks, then it will throw this error while syncing and it can block the sync as it will identify the line space as a new item you have added which does not have a Name or Quantity. It will quit the sync and there is a chance of missing transaction on the other side if you did not check the records in Salesforce.
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