falko, this morning maybe i've resolved the problem.
Ill'wait to "sing victory" as in italy we are usde to say.
The problem maybe it's due to the option of zywall usg200 called "Enable link sticking"
This option permits to mantain the same session active to a long time.
This option was disabled and so I think pcs change session every time after authentication so they don't receive data. I'll inform you during the day about the reactions of clients.