I have not yet solved my problem, it was just on hold.
Removing all custom code from my child theme made no difference.
I tried deactivating the plugins one by one, and first when ALL other plugins than woocommerce is disabled, then the countries defined in selling locations appear in the checkout form.
I tried deleting woocommerce and installing it again, also no change. I however didn’t remove all data, so when I installed it again everything was the same, so it makes sense that it made no difference.
Any other idea than removing woocommerce + data?