Thank you for your patience.
I actually meant that I get the complete nested array of a repeater with get_field() with field keys instead of field names.
After researching for a while, I realized that the last parameter format does exactly that. This gives me field keys instead of names.
I have now found a solution that works quite well:
1.) I use acf_get_field_group() to read in the field group that I would like to display in my boxes.
2) With acf_get_fields() I get the corresponding fields in an array and can cycle through them.
3) With acf_get_field() I get the field object in the loop that I need later to display the field.
4) With get_field() and the last parameter set to false, I get the representation of the data that has already been saved for this field
5) I pass all this to acf_render_field_wrap() and so get the representation of the control in my box
Finally, I need to make it possible to have several boxes with the nested fields at the same time. At the moment it is only possible with one box.
