Snow Leopard Server Install Bug
If you have more than one ethernet port connected during install you won’t be able to verify the serial number. The installer binds the serial number to the MAC address of one of your ethernet ports. If you have more than one active during install it will ‘think’ that it’s already bound to the other one and will fail to validate.
The solution is to unplug all but one ethernet port through the install, and to run Software Update at least once afterward, before plugging the other ports back in.