Thank you in the meantime for your support.
I found it, if I turn off the network traffic scanner, the ping works.
I can disable the option from Configuration > Advanced Configuration > Detection Engine > Network Traffic Scanner > "Enable Network Traffic Scanner" on/off switch.
However, I don't have an option to modulate this choice, either I deactivate all traffic or enable everything, I would like to be able to only deactivate the ping control towards the address of my home automation server.