Hi @tadej
Could you please make sure that the issue is not related to the max_vars variable? This page should give you more idea about it: https://www.advancedcustomfields.com/resources/limit-number-fields/.
Also, could you please make sure that the issue is not the field name? This page should give you more idea about it: https://www.advancedcustomfields.com/resources/field-value-wont-save/.
If you are using the Local JSON feature, could you please disable it for a while?
If that doesn’t help, 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. This will make sure that no other plugins that cause this issue.
Thanks 🙂
