Hi Jaques, we need to see how the dedicated page is configured and how the user profile setting has been compiled to be of help. Can you share screenshots of the page you reported which page is? Is the page set ad dedicated page on main settings? That page contains the [newsletter] shortcode? Are there warnings on the admin side?
When I subscribe your blog, I end to the page
which is probably the dedicated page set on main settings and which has been trashed. If you’re running newsletter 5.5.1 (which version is installed) you should have a notice about that page not being correctly configured.