The preview could not load due to the iframe response being incomplete…

I get the above error message when trying to edit certain pages (but not others, even when the content / page structure is very similar).

Hello @neilandroid,

Thanks for writing in!

I have checked the page and there is an error 403:

       Frame missing CORNERSTONE_FRAME signature. This could mean WordPress did not shutdown properly or this URL could be cached even though Cache Control headers are present.
(anonymous) @ app.6f62d0f.js:2
Promise.then (async)
(anonymous) @ app.6f62d0f.js:2
Bh @ react-dom.min.js?ver=16.13.1:126
Dj @ react-dom.min.js?ver=16.13.1:162
unstable_runWithPriority @ react.min.js?ver=16.13.1:25
Da @ react-dom.min.js?ver=16.13.1:60
xb @ react-dom.min.js?ver=16.13.1:162
(anonymous) @ react-dom.min.js?ver=16.13.1:162
U @ react.min.js?ver=16.13.1:16
B.port1.onmessage @ react.min.js?ver=16.13.1:24
       POST 403 (Forbidden)

Please enable your debug mode to check for the errors:

It could also be caused by a plugin conflict, amongst other things. Please check out this troubleshooting article here and follow the instructions for the following sections (where appropriate):

Best Regards.