The group User had only the permission to read, not to create or to change a page. The group Editor (https://christipedia.miraheze.org/wiki/Speciaal:ManageWikiPermissions/editor) however had the right to change pages. It seems this distinction has changed: although the permissions of the group User seem to be set correct on the permissions form (https://christipedia.miraheze.org/wiki/Speciaal:ManageWikiPermissions/user ), in reality every visitor (not logged in user) can change a page or create one and save it. I discovered it today.
I understand now that the group User is not the default group for visitors, it is the group * that is the default visitors group. So I misconceived it. However, I think remembering that I could not create of modify a page when logged out.