(Prologue; because of my wife's sudden "Christmas clean the house" whim, I had to move some of the dust collecting computers, including disconnect them from the network etc...)

After reconnecting one of the computers to the switch, it couldn't find the dhcp server (well, couldn't find any other machine...)
The only change was that I used another cable than before

Solution:
Changed Link Speed & Duplex from Auto to 10Mbps/Half Duplex and the connection was working again
Can someone explain why?
(Different type of cable => different speed properties? )