I am sorry for the delayed response.
Could you please increase the post, get and request max vars too? This page should give you more idea about it: https://www.advancedcustomfields.com/resources/limit-number-fields/.
If that doesn’t work, could you please check if you have the Local JSON feature activated? Please take a look at this page to learn more about this feature: https://www.advancedcustomfields.com/resources/local-json/.
Also, could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
If still no luck, could you please ask your hosting if there are error messages logged?
Please let me know how it goes 🙂
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