Hello John, everything's ok now. I finally discovered the culprit behind my miseries! Someone in our network connected a router into the LAN (same model). I discovered the difference between the two via the mac address displayed in the setup. They tried to use the router as a dumb switch, unfortunately, they didn't even consider reconfiguring its setup first. Once I configured it's IP and disabled DHCP, everything went back to normal.
Thanks again for your help!