hiperlink I can see that you've received a reply from support of this. I'm going to share the response here as it will help any other community members who may have received the email:
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This email doesn't explain the problem but in other cases it's because mysql was open to the internet.
If you don't want this, you can block the port (3306) in the Hetzner firewall (if available) and just allow ports for the servers you want to connect - as a minimum you need to allow the control panel server to connect for phpMyAdmin to work.
You can also do it with ufw on the server itself:
ufw delete allow 3306
# allow your authorised IPs, repeat for each IP you want to allow
ufw allow from 1.2.3.4
ufw allow from 2.3.4.5
....
