Hi Peter,
Access to the sections is restricted because in the menu items, you have set the Access to Werkzoekende en -aanbieder. So, public visitors can't access these menu items.
You must first make these menu items public.
Then, in order to allow registered users only to post questions, you must go the component Configuration > Permissions:
1. For Public users, set the Create permission to Denied.
2. For Registered users, set the Create permission to Allowed.