We have seen this in a couple of scenarios:
2. When you have a column in the list that is set to only have unique values in it. The error is hidden by the Nintex Form.
You could try exporting the form (so you have a copy), deleting the Nintex form, try to add the item with the same values with the out of the box SharePoint dialog to see if there are any errors.
That should do it. I don’t currently have access to the form to try it – 30 day trial ran out – but will keep that in mind for next time.
616-451-1926 or cell: 616-304-2337