Today I upgraded my BIOS in the hope to solve various issues. When I rebooted the machine, it didn't get an IP address through DHCP: the Ethernet MAC address has been changed by the BIOS upgrade.
Now compare the old and the new Ethernet MAC addresses:
old address: 0:1a:4d:60:72:e0 new address: e0:72:60:4d:1a:0
Time to laugh...