this is what i've discovered so far:
-i've got the problem of failed login on diablo only behind a nat (quite obvious), the same settings (discover public address enabled) works fine on a plain connection, no drop after 30 secs
-on chinhook everything works fine, but if i disable the discover public address the call will drop after about 30 secs
-on diablo the login works only with discover public address disabled but this make the incoming calls drop after about 30 secs

i'm waiting to come back from holidays to set up the test environment to sniff the packets