Hi @dec
It’s possible that you have an issue with the max_vars variable. To fix it, kindly check this page: https://www.advancedcustomfields.com/resources/limit-number-fields/.
If that doesn’t work, it’s also possible that there’s a conflict on your site. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) 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 joy, could you please check the error log on your server. The error log usually has more information regarding that issue. If you don’t know where to find it, kindly get in touch with your hosting provider.
Also, could you please share the JSON export file of your field group so I can test it out on my installation?
Thanks 🙂
