Back again with the Raspberry Pi working. It turned out that my Router was the culprit. Connected directly to the router, the RPi refused to establish a link. I have a Netgear 8 ports Gigabit switch connected to the Router and once I moved the cables around and connected the RPi to the switch, the link was established and everything started to work.
Never knew this could actually happen, but it did. So, interesting information: Raspberry Pi 4 connected to a Netgear D7000 -> doesn't work!
This topic can be closed now.