Originally this message reads in German:
Es wurde ein Nicht-UTF-8-Zeichensatz für MySQL/MariaDB gefunden
I get this notification each time I switch to the news app.
This happens the first time I upgrade to NC 12.0.5 and installed newly the news app.
I followed already the advice and changed the column settings of all news tables in phpmyadmin with, e.g.,
ALTER TABLE
oc_news_items
CONVERT TO CHARACTER SET utf8mb4
COLLATE utf8mb4_unicode_ci;
After that, I stopped and started mysql and apache2. But the result is all the same.