Продолжительные скачки с бубном и завывания - наше всё.
Решение присмотрел на иностранном форуме:
For the initial installation, you may wish to attempt it via a direct Ethernet connection.
1. On the Windows computer, set its network address to 169.254.1.1 subnet mask 255.255.0.0.
2. Connect a straight through Ethernet cable between the computer and the DS108j.
3. Power the Synology server down and wait 30 seconds, then power it back up.
4. Launch the Synology Assistant and attempt your installation (given the address range you will need to use step by step installation).
5. Set the Synology Server to use automatic (DHCP) addressing.
Мне помогло.