Thank you for your quick answer, Stefano.
As “Newsletter dedicated page” I have selected the page where the newsletter-form is included (“my-newsletter-page”).
Installed plugins: Contact Form 7, Duplicate Post, Newsletter, TinyMCE Advanced
After click to “unsubscribe” this URL comes: /?na=uc&nk=2-974b501a11&nek=10 (white page).
Then it turnes to this URL: /my-newsletter-page/?nm=unsubscribed&nk=2-974b501a11 . It shows the normal site with the newsletter-form, without an additional massage.
What can I do, to make the goodbye message show up?