Hi, I solved my problem. It was a bit tricky, but I did :)
Briefly: I had to re-flash the firmware while my linkstation was in EM mode (http://buffalo.nas-central.org/wiki/EM_Mode).
In some discussion topics I read about "clicking twice the reset button" to enter the EM mode. This not worked for me, so I used the apc_commander (http://buffalo.nas-central.org/index.php/Open_Stock_Firmware#Getting_Console_.28Telnet.29_Access_with_acp_commander).
So the success story steps were:
1) dowload the acp_commander.jar
2) reset the linkstation (pressing the reset button for at least 15 seconds): this resets the admin password to "password" and sets the IP to the the standard 192.168.11.150.
3) configure PC LAN settings to use a static IP like 192.168.11.8 / 255.255.255.0
4) directly link the PC to the linkstation using a standard/or even a crossover LAN cable
5) disable all firewall softwares (yes, also the Windows firewall)
6) to enter the EM mode, run the apc_commander: java -jar acp_commander.jar -emmode -t 192.168.11.150
7) reboot the linkstation
8) wait for linkstation reboot completion and then access the web login page (http://192.168.11.150): an alert should confirm you that the linkstation is now in EM mode
9) use the firmware utility to flash the firmware (I use version 1.15 for my LS-320GL)
10) access the web login again: now it should work but... in japanese language! Don't panic, use the apc_commander again: java -jar acp_commander.jar -gui 1 -t 192.168.11.150, where "-gui 1" sets the English language (use "-gui 2" for German)
11) at this point you should enter the web login page using admin/password and see all the shares.
I hope this story can help someone else with my same problem.
Thanks,
Gaspare