I still don’t have access to your site. Tried twice to reset password with ‘support [at-sign] role-editor.com’ and did not get any message from your site.
Empty testimonials list – check if any edit restrictions was set for Klant role or a user with this role.
Redirection to dashboard on add new testimonial – do you block some menu items for Klant role with ‘Admin menu’?
Media – check if related permissions are granted to a role under “Custom Post Types -> Media” group.