Hi
@P1LGRIM Problem solved on both machines by generating a new Network "Location" and deleting the old one in System/Preferences/Network. Thereafter some minimal network configuration was required, and that was it. Both machines now handle "Wake for network access" again the way they have always done.
The network link from the NIC to the switch port, which serves the particular NIC, remains alive while the machine is asleep. The LED on the NIC as well as on its respective switch port also remain lit during sleep.
Thanks for your assistance throughout. Will now mark this thread as [SOLVED] hoping that others with the same problem have it easier to rectify.
Greeting