An embedded page at formso365.nintex.com says:
There are some problems with the design of this form and this is causing some errors. Please contact the designer of this form to rectify these.
The form is reporting the following errors:
<control name> is not defined
This error persists even after deleting the form and creating a new version with the default SharePoint form.
- In SharePoint designer, navigate to the site > All Files > Lists > ListName > Item.
- Delete the NFForm.xml file.
- Navigate to https://tenanturl/site/NintexFormXml.
- Find and delete the file that contains the matching List GUID.
- Create a new form in the designer.