Nintex Forms O365 access is denied for external user when a document in document library is shared with edit permission. The document is shared via email. When the user clicks the link and sign in using his Microsoft account and he is able to see the document. But when he clicks to edit the properties the access for Nintex form is denied.
For this, you may need to be sure the user has contribute or edit permissions to the list that host the Nintex form. Without that the user will not have access to see the form as you may be experiencing.
Let me know if that helps.