node/ID?_format=json gives {"message":"Not acceptable format: json"}

I am using Drupal 8.2.0-rc2. I have HAL, HTTP Basic Authentication, REST UI, RESTful Web Services and Serialization enabled.

In REST UI I have configured:
Content enabled
* hal_json selected
* json selected
* xml NOT selected
* basic_auth selected
* cookie selected

All other resources are disabled.

Node 68 is of self created content type “Question”.

http://my_domain/node/68?_format=json gives {“message”:”Not acceptable format: json”}. Why?

I was 1) logged in as admin and 2) not logged in. Same results.
1) ADMINISTRATOR has all permissions automatically
2) ANONYMOUS USER has the following permissions checked
* View all revisions
* View published content
* Form_vraag: View revisions

Anyone any idea what’s the problem?

Drupal version: