CSV-import: I want to update a custom free text field on sales order. On some orders, I want to write something in the custom field, in some orders I want to NULL the custom field. I want to do both actions in the same CSV import. I have tried to give empty value and to write "NULL" in the field but none of them work. "NULL" gives error, empty value doesn't do anything.

Make sure the Overwrite Missing Fields is checked, in Step 2 Import Options under the Advanced Options drop down - then you can leave the fields on the CSV blank

great, thank you!