After upload the monitor displays:
Booting
Ready
IP address: 192.168.178.20
... so far so good.
As far as I understood the process, this sketch sets up mDNS, but Sniffer just lists ARP as service .... and I do not get the OTA port displayed in Arduino IDE ...
I turned off all firewalls ....
Thanks for help, Markus
Update >> It was the firewall ... I am using a company computer with managed SEP installed. After uninstalling SEP the port showed up -just disabling was not enough- now after reinstalling SEP, the port stays, and can be used after disabling SEP temporarily. There is nothing to do on the fritz box (fritz!box), and nothing to change/add/install with Bonjour.