After updating WP to 4.9, X to 5.2.5 and Cornerstone to 2.17 I can confirm having the following nag screen when trying to edit pages with cornerstone:
“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 issue disappears when Polylang is disabled, so it seems to be still related to it.