The other day the network went down suddenly. Research revealed that the core switch had obviously died. It is a Netgear GS116E 16-port gigabit switch. Only the Power LED and the 100Mbps LED of the second LAN port were on. Resets to firmware defaults did not change anything. Further look at the Power LED showed that it was slightly dimmed flashing as if power supply was having issues powering the switch.
Using a lab power supply feeding 12V with 2A to the switch immediately brought it back to live. And it was even having the stored settings still. So it was definielty an issue of the power supply.
# Fixing the Power Supply
Opening the unit immediately revealed a capacitor with lifted cap. It is rated 680µF at 25V (and 105° C temp) and seated directly at the DC output. Measuring the ouput in idle showed 14.5V. So a repelacement with 16V looked ok though it only had 470µF. But tests showed that it was obviously sufficient. The switch is back up and running with fixed power supply.