API not returning attribute/id/terms

We have;
Wordpress 5.2.1 multisite
theme: Twenty Nineteen
Woocommerce 3.6.4
No other plugins

the call to:
https://oursite.com//wc-api/v3/products/attributes/8?consumer_key=ck_9d7e….
returns;
{“product_attribute”:{“id”:8,”name”:”Kleur”,”slug”:”pa_kleur”,”type”:”select”,”order_by”:”menu_order”,”has_archives”:true}}

But if we then call:
https://oursite.com//wc-api/v3/products/attributes/8/terms?consumer_key=ck_9d7e….
the return is;
{“errors”:[{“code”:”woocommerce_api_invalid_product_attribute_id”,”message”:”A product attribute with the provided ID could not be found”}]}

All other endpoints seem to work so far.
Could you please give us a hint as to where we make a mistake in the calling?
thanks!