At last I’ve been able to solve the trouble, so I would like to share the solution in case anyone should have the same issue.
Downgrading/upgrading the firmware version did not help at all.
Nevertheless, the PWT has always been being connected to the local network and, from a certain moment on, something got wrong with the “handshake” between the network router and the PWT during the bootstrap process (the PWT DHCP mode is “on” by default, so the router tries to assign a “free” IP address to the PWT when it “asks” to join the network).
First of all, I discovered the bootstrap process went fine whenever the ethernet cable was disconnected from the rear panel socket. Reconnecting the ethernet cable once the starting sequence has ended, the PWT gets the IP address and everything runs ok.
I’m assuming the defect is on the PWT side and not the router/local network side because many other devices are connected with DHCP mode and they have no issues when restarting. Among others, I checked of course the PWT gets an IP address not conflicting with IP addresses of any other network device.
The solution I found was to change the PWT default network configuration disabling the DHCP mode and assigning the PWT a “manual” IP address (this can be done via browser writing the PWT IP number in the top address bar). On router/network side the PWT “manually assigned” IP address has to be marked as reserved, so no other devices with DHCP mode can get the same IP number.
Doing so, the PWT bootstrap process runs smoothly without the need of disconnect/reconnect the ethernet cable.