Hi @joostvanhoof
Just to clarify, this was a WP export data, and then a WP import data?
Can you check that all the custom field values exist in the database and that for each value, there is a hidden value with the same field name but with an underscore before it.
This ‘reference’ value is explained here and is important for ACF to format the value before returning it:
http://www.advancedcustomfields.com/resources/functions/update_field/
Thanks
E
