I lost a part of message i wrote upper in the post.
During my firts attempt to upgrade USG via console (the second one, as the first from WEB GUI failed ...), i reported some error messages because of dead configuration's settings (AD no more actives, some test SSL with no workin' elements and so on.) on which updating was stopping.
I removed them from startup-config.conf and updating went on, till zysh crash.
Maybe i got other "wrong" settings for the 3.00 requirements.
Dunno if i explained well what i mean.