I seem to have the same issue described here: [The preview could not load due to the iframe response being incomplete. This is most often related to a plugin conflict or customizations introducing a PHP error.
The weird thing is that it only happens on my Contact Us page, and even when I tried creating a new page named Contact it gave me the same issue.
I followed the steps provided by your staff in this post and the issue persists. Some assistance with the matter would be great.
Thank you!