When uploading a document to a document library in SharePoint Online that has a Nintex Form published, Internet Explorer presents a window that states "This content cannot be displayed in a frame".

 In Internet Explorer, please navigate to Settings (Cog in top right) > Internet Options > Security > Trusted Sites > Sites and add the following sites:

*.office.com
*.microsoftonline.com

You may need to close and reopen Internet Explorer for this to take affect.

"This content cannot be displayed in a frame"

This appears to only occur when there is a SharePoint Column that is 'Required'.