With the help of D-Link tech support, we found that this is related to the DHCP implementation in the switch.
As soon as the switch is configured in DHCP, i need to perform a soft reset (holding reset button for 2 sec) in order to regain connectivity after a power loss.
I configured the switch with a static IP and now it's ok, no need to soft reset anymore.
I'm pretty sure I already tried that but the d-link tech also pointed me that the switch does not save to NVRAM automatically. this mean if you change the IP of you switch, you need to make sure to go back in UI with the new IP to save the configuration to NVRAM. If not, IP settings will be lost at next AC loss.