That was exactly the problem Stefano. I have changed the setting and now all messages are displayed as expected. Thank you so much for your swift and accurate diagnose.
Best Regards,
Jos Teunissen
Thanks Stefano. URL is: https://stadslandbouwzeist.nl/newsletter/
Should you wonder: the public page only has the code [newsletter] in it. Additional text is added as custom text in set-up newsletter messages.