1- Open the Nintex Form in the Nintex Form Designer.
2- Click on the "Export" button on the ribbon menu and export the Form as Form.XML file on the local drive.
3- Now, click on the "Delete" to delete the Form. Please note that any data or items on List that was added to the SharePoint list will "not" be deleted, as the list items are saved on the SharePoint List, which mean on the SharePoint content DB.
4- After you deleted the Nintex Form, now click on "Import" the same Form.XML from your local drive and attempt to Publish the Form again. That should work.