It’s kind of frustrating that this is still open. I posed the cause and solution over a year ago and it was deleted.
I posted it here, and on the WordPress Plugin area, both removed.
It is fixed by adjusting the max_input_vars.
What I had originally suggested was moving the Nonce to the beginning of the JSON payload and then to throw an invalid JSON error which is probably a more useful and accurate error that an invalid nonce.
Because the payload is too large the nonce isn’t captured by the server.
I have the same issue,
New site. ACF is installed and my custom fields are imported from the themes acf-json folder.
The ACF fields were exported from one site and imported into the other.
I exported my pages from the old site, imported (standard WP Import) them into the new one.
When I view the page on the backend I can see everything displaying correctly but nothing shows until I update the page.
I don’t know if it helps, but none of the imported pages have content in the main body, they are 100% ACF pages.
