The conclusion ...
It is the Kea DHCP server on pfSense in combination on the Mac using “DHCP with manual address”, which gave me a headache.
Now everything is fine again and I’m happy that no Hardware had to be replaced, what I can’t afford anyhow.
So hopefully for the week, I can start now with my “last week projects”.
Did today 9km in 74 minutes, it felt quite fast, I’m walking, not running 😉