Test notifications goes wrong

CMK version: 2.4
OS version: Debian 12

Error message: Network error

Output of “cmk --debug -vvn hostname”: (If it is a problem with checks or plugins)

As soon as I go to test notifications and want to send a notification by email to my registered address, this page appears.
My firewall is open for testing, so I don’t understand where this error could be coming from.

Furthermore, I am not receiving the notification by email. However, it says that it has been sent.

Do you have any idea what could be causing this?